SOCIAL-406 Allow adding connections to authenticated users via SocialAuthenticationFilter

Fixes [SOCIAL-406](https://jira.springsource.org/browse/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(..)