Merge pull request #278 from LeadSift/master

Adding Bitly as an OAuth2 Provider
Properly giving Bitly contributor credit
Added social_account_added signal (closes #272)
Improved SocialAccount admin usability
Python 2 to 3 fix.
Fixed DeprecationWarning errors in tests.
Python3 fix for map() in admin.
Add `sociallogin` to `user_logged_in` signal (closes #279)
Added oauth2 CSRF protection (closes #266)
oauth2 csrf, take two
Moved social account disconnect validation to adapter, added social_account_removed signal