Add new room invitation method

This adds a new method to Candy.Core.Action.Jabber.Room called "Invite" that allows sending MUC invitations to XMPP users.
It works like this.  The [Strophe docs for `#send`](http://strophe.im/strophejs/doc/1.1.3/files/strophe-js.html#Strophe.Connection.send) state that it takes a `Strophe.Builder` object, which this is. The [Strophe docs for `#tree`](http://strophe.im/strophejs/doc/1.1.3/files/strophe-js.html#Strophe.Builder.tree) state that it returns something suitable for `#send`. The way I read it, you can use `#tree`, but it shouldn't be required if you have a `Strophe.Builder` object.