SOCIAL-406 Allow adding connections to authenticated users via SocialAuthenticationFilter

Fixes SOCIAL-406 and #95

- moved `response.sendRedirect(((SocialAuthenticationRedirectException) failed).getRedirectUrl());` out of SocialAuthenticationFailureHandler (thus making it obsolete) and into SocialAuthenticationFilter.attemptAuthentication(..)

- use Connection instead of ConnectionData as argument do addConnection(..)