Issue #137 - detect sync gateway server version

https://github.com/couchbase/couchbase-lite-android/issues/137
Closes issue #42

https://github.com/couchbase/couchbase-lite-java-core/issues/42
Issue #137 - do POST request to _changes when possible

https://github.com/couchbase/couchbase-lite-android/issues/137
Issue #137 - only set limit iff mode == long poll.  fixes unit test failures.

https://github.com/couchbase/couchbase-lite-android/issues/137