Fixes for the new XEP-0045 plugin

I've been using the `new_muc` branch for some time in a project of mine and I have some fixes to submit for it:

* Fixed a bug in the `_add_joined_room` method where it erases the roster for the room, clearing the client itself from the roster (ie the client is added to the roster then the roster is reset). Ashfire908@354789d
* Various typos and error fixes. Ashfire908@639d9b6
* Fixed potential issue where the wrong room join failure could be responded to. Ashfire908@c545be6
* Fixed mediated invites. Ashfire908@36ddb6c