Update Providers\Facebook.php

* Added 'username' which is part of the "Public Profile" based on
http://developers.facebook.com/docs/reference/login/public-profile-and-friend-list/
* Added 'coverUrl' which attempts to retrieve the url to the cover image
Update Facebook.php

Updated case to `coverURL` from `coverUrl`
Updated Providers\Facebook.php

Removed auto fetching of cover url image instead only provide url to the
cover's info xml and let the user retrieve the image from a separate
function call.
Update Providers\Facebook.php

Tweaked variable name to better reflect its content.
Merge pull request #126 from maximerassi/master

Updated Providers\Facebook.php for getting cover photo.