This neatly allows to pass the room object passed to the "invited" event
directly to :join_room(). This way the user does not need to know that
rooms have a "room_id" attribute.
This allows hooking to both room join and room leave events at the client
level, which apart from being more orthogonal, may be more convenient
sometimes.
This removes the currently unused "limit" parameter from
client:login_with_password() and client:register_with_password(), and adds a
new "no_sync" parameter. When the flag is set, the call to client:_sync() is
skipped. This can be useful for small scripts which do not need to do a sync
after login, yet they would rather user the higher level "matrix.client"
instead of the low level "matrix.api". For example, a simple script to send
a message to an already-known room given its ID could skip the sync in order
to be faster, without needing to revert to using "matrix.api".
