Make purgeRevisions() public so it can be called from router package.
While implementing do_POST_Document_purge() I was getting a nullpointerexception since this List was not initialized
Implement do_POST_Document_purge() to fix issue #76

https://github.com/couchbase/couchbase-lite-android/issues/76
Rename from getPullReplication -> createPullReplication per API spec.  Ditto for push replication.
Remove getReplications() as it is not part of the API spec