Set reference to roster contact on the message and chatbox object and
listen for changes to the nickname.
Currently, because chat boxes are fetched and initialized before the
roster, messages and chats are repainted with the correct display name
only later, causing a "flash" effect.
Ideally we would only initialize the chat boxes after the roster
contacts have been fetched, but this is currently not easily possible
because we need the control box to render before everything else.
When no status message is in a presence stanza, the value gets set to
undefined, so we might as well make this the default value to avoid
unnecessarily triggering change events.
Avoid `An 'url' property must be specified` error by properly clearing
presence data upon teardown and then resetting the browserStorage upon
reconnection.
Store contact resources in a Backbone collection
* Return the promise, so that we can wait.
* `sendContactAddIQ` now returns a promise and doesn't take a callback
* Don't call `destroy` on a contact that's already been removed.
