Added helper methods to set, release and check user_id in zmq_target.
Fixed typo in calling _prepare_u1db_action.
Removed local U1DB trunk files. Now installed as a dependency.
Fixed database opening issues. Now, u1db can open a database based off a user_id, provided that database is already present.
Updated requirements.txt
Fixed data being packed into message structures.
Changed type of doc_rev to string. Modified tests accordingly.
Added workaround to find out last_known generation and trans_id before validating them in prepare_for_sync_exchange.
Fixed bugs in sync. Current BUG is that target returns the doc just received from source.