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).

specification/appendices.rst

@@ -1,3 +1,17 @@
+.. Copyright 2015 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.
+
 Appendices
 ==========