Preliminary tests for $firebaseAuth

I took a stab at it. Let me know what you think.
BTW. There's a missing `,` in one of the code samples in those docs you linked to.

```javascript
  "user_id": "<USER-ID>"   //HERE!!!
  "oauth_token": "<ACCESS-TOKEN>",
  "oauth_token_secret": "<ACCESS-TOKEN-SECRET>",
```