Add a license to the spec

We're licensing hte spec under ASLv2. Add the LICENSE file, and add the
short-form to as much of the source as is practical right now (adding it to
json source is a massive pita).

api/client-server/sync.yaml

@@ -1,3 +1,16 @@
+# Copyright 2016 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
 swagger: '2.0'
 info:
   title: "Matrix Client-Server sync API"