Update 3pid invite spec

This takes into account:
 1) That finding the existing servers of a room is hard
 2) Federation

event-schemas/examples/v1/m.room.member

@@ -3,7 +3,12 @@
   "content": {
     "membership": "join",
     "avatar_url": "mxc://localhost/SEsfnsuifSDFSSEF#auto",
-    "displayname": "Alice Margatroid"
+    "displayname": "Alice Margatroid",
+    "token": "pc98",
+    "public_key": "abc123",
+    "key_validity_url": "https://magic.forest/verifykey",
+    "signature": "q1w2e3",
+    "sender": "@zun:zun.soft"
   },
   "state_key": "@alice:localhost",
   "origin_server_ts": 1431961217939,

event-schemas/examples/v1/m.room.third_party_invite

@@ -0,0 +1,14 @@
+{
+  "age": 242352,
+  "content": {
+    "display_name": "Alice Margatroid",
+    "key_validity_url": "https://magic.forest/verifykey",
+    "public_key": "abc123"
+  },
+  "state_key": "pc98",
+  "origin_server_ts": 1431961217939,
+  "event_id": "$WLGTSEFSEF:localhost",
+  "type": "m.room.third_party_invite",
+  "room_id": "!Cuyf34gef24t:localhost",
+  "sender": "@example:localhost"
+}

event-schemas/schema/v1/m.room.member

@@ -1,7 +1,7 @@
 {
     "type": "object",
     "title": "The current membership state of a user in the room.",
-    "description": "Adjusts the membership state for a user in a room. It is preferable to use the membership APIs (``/rooms/<room id>/invite`` etc) when performing membership actions rather than adjusting the state directly as there are a restricted set of valid transformations. For example, user A cannot force user B to join a room, and trying to force this state change directly will fail.",
+    "description": "Adjusts the membership state for a user in a room. It is preferable to use the membership APIs (``/rooms/<room id>/invite`` etc) when performing membership actions rather than adjusting the state directly as there are a restricted set of valid transformations. For example, user A cannot force user B to join a room, and trying to force this state change directly will fail. The token, public_key, key_validity_url, signature, and sender properties either must all or none be set - they are set if the invite was an ``m.room.third_party_invite`` event, and absent if the invite was an ``m.room.member`` event.",
     "allOf": [{
         "$ref": "core-event-schema/state_event.json"
     }],
@@ -21,7 +21,27 @@
                 "displayname": {
                     "type": ["string", "null"],
                     "description": "The display name for this user, if any. This is added by the homeserver."
-                }
+                },
+		"token": {
+		    "type": "string",
+		    "description": "A token which must be correctly signed, in order to join the room."
+		},
+		"key_validity_url": {
+		    "type": "string",
+		    "description": "A URL which can be fetched, with querystring public_key=public_key, to validate whether the key has been revoked. The URL must return a JSON object containing a boolean property named 'valid'."
+		},
+		"public_key": {
+		    "type": "string",
+		    "description": "A base64-encoded ed25519 key with which token must be signed."
+		},
+		"signature": {
+		    "type": "string",
+		    "description": "A base64-encoded signature of token with public_key."
+		},
+		"sender": {
+		    "type": "string",
+		    "description": "The matrix user ID of the user who send the invite which is being used."
+		}
             },
             "required": ["membership"]
         },

event-schemas/schema/v1/m.room.third_party_invite

@@ -0,0 +1,37 @@
+{
+    "$schema": "http://json-schema.org/draft-04/schema#",
+    "type": "object",
+    "title": "An invitation to a room issued to a third party identifier, rather than a matrix user ID.",
+    "description": "Acts as an ``m.room.member`` invite event, where there isn't a target user_id to invite. This event contains a token and a public key whose private key must be used to sign the token. Any user who can present that signature may use this invitation to join the target room.",
+    "allOf": [{
+        "$ref": "core#/definitions/state_event"
+    }],
+    "properties": {
+        "content": {
+            "type": "object",
+            "properties": {
+                "display_name": {
+                    "type": "string",
+                    "description": "A user-readable string which represents the user who has been invited. This should not contain the user's third party ID, as otherwise when the invite is accepted it would leak the association between the matrix ID and the third party ID."
+                },
+		"key_validity_url": {
+		    "type": "string",
+		    "description": "A URL which can be fetched, with querystring public_key=public_key, to validate whether the key has been revoked. The URL must return a JSON object containing a boolean property named 'valid'."
+		},
+		"public_key": {
+		    "type": "string",
+		    "description": "A base64-encoded ed25519 key with which token must be signed."
+		}
+            },
+            "required": ["display_name", "key_validity_url", "public_key"]
+        },
+        "state_key": {
+            "type": "string",
+            "description": "The token, of which a signature must be produced in order to join the room."
+        },
+        "type": {
+            "type": "string",
+            "enum": ["m.room.third_party_invite"]
+        }
+    }
+}