Fixed group object bugs

* Fetch was running into an issue where if an error was encountered and the logging setting on the client was false it would silently fail. We use fetch in group creation to determine if it exists. If it doesn't exist that error condition occurs and the group isn't created the function just hangs waiting for the callback.

* The callback for ```Usergrid.Group.prototype.add()``` was incorrectly implemented. It wasn't passing back the list of group members as the third argument as expected. I've corrected this issue.