Added method get_user() for several providers

Hi there!

This is an awesome project and thank you for all of the work and effort that you have put into it.  I have added a new method called get_user() for a few of the provider files.I felt that the method get_user_info() (See: OAuth2/Provider/Facebook.php) may be a bit limited when retrieving the full provider response.  This method returns the full contents of the provider's response in the form of an array when retrieving data from Facebook, Google, GitHub, MailChimp, and WindowsLive.

If you have any questions or concerns, please do not hesitate to get back to me.  i'll be more than happy to give a helping hand.

Thanks,
Koufax