Model does not 'resume synchronization' after a returning to a room

[mattcreager]

GoInstant cleans up a rooms listeners when room.leave() is explicitly called. GoAngular should probably:

1. Clean up listeners
2. Empty the model
3. Emit an event
4. Listen for a join event, on the associated room
5. Emit on join
6. Resume synchronization

cc: @MaheshCasiraghi