Adding support for Firebase events and $off / $unbind methods.

Cheers!
While creating a collaborative app in AngularJS I had naturally decided to use AngularFire, since it's awesome. :)
However, I have soon found  out that `change` and `loaded` events are not enough for me, especially that they have not provided the `snapshot` and `prevChild` data for handlers.

The pull request consists of precisely this functionality, plus `$off(type, handler)` (unbinding a specific handler from an event) and `$unbind(type)` (unbinding all handlers from an event).

The test suite passes BUT I have not added any new tests (I'm 'testing' the events continuously on my app, so did not have the need) - I can do that if it's a necessity- but at a later moment, I'm currently under considerable time pressure.

Hope you find this PR of value. :)