Add url option to FirebaseStore

[brianneisler]

This would allow for passing in a full firebase url with the https:// already prefixed to the host.

var options = {
  // The URL you were given when you created your Firebase
  url: 'https://connect-sessions.firebaseio.com',
};

Might also be worth making the host option a bit more forgiving or at least providing a more specific error message. Right now if you pass a full firebase url as the host you will receive back a generic error from the firebase library.

FIREBASE FATAL ERROR: Cannot parse Firebase url. Please use https://<YOUR FIREBASE>.firebaseio.com

This error at least points you in the right direction but would be nice if it told you exactly what was wrong.