additional showOAuthLoginControllerFromViewController: method added that takes in completion block

I've added an additional showOAuthLoginControllerFromViewController that takes in a completion block parameter so that users of the library can actually find out if the user successfully authenticated. Currently there is no easy way to find out if the user pressed cancel or if they authenticated successfully.

On a side note the completion block is called with the inverse of the return:
self.completionBlock(!ret);
This is because you consider 0 a successful login while 1 is a failure, but these correspond to FALSE and TRUE respectively contrary to what you would expect.