Presence API

Changed the presence API from master. Now a plugin has to define : 
def callback_presence(presence):

And will be callbacked with a presence object.
The presence object defines if it is relative to a contact or a contact in a chatroom + alias (nickname).
It also standardizes the statuses ONLINE, OFFLINE, AWAY and DND across the backends.

Implemented the callbacks for XMPP and TOX to begin with.