Add createUser - issue #57

This seems to work in the cases I could come up with.  Fairly straight forward and consistent with the other angularFireAuth functions.
Add createUser - Issue #57 part 2

Decided the callback should probably be optional, as someone could be doing work in the $broadcast of the login event instead of wanting to pass a callback.
Merge branch 'patch-2' of https://github.com/fronn/angularFire into fronn-patch-2

Conflicts:
	angularFire.js