Split ZMQSynchronizer._sync and some tests. More work to be done on this.
Added tests for zmq_target.ZMQSyncTarget
Added tests + fixed bugs for zmq_transport.client.sync module; Used a lot of mocks!
Added tests for zmq_transport.u1db.sync
Fixed bug in u1db/zmq_target.py that was introduced as a typo in commit 0a7caa13de9e9264ec8070821d68fb5efe6b9e32 and fixed bug in client/sync.py that was introduced while splitting the _sync method.