Added support for Gravatar.
Set $request->expect for expected HTTP code > 300.
HEAD requests return http status code
Replace `@see http://` to `@link http://` in doc blocks
Small fix gravatar
Added Person::getId() common interface method
Implemented for Facebook, Google, LinkedIn & Twitter
Always expect a 404 for Gravatar
Gravatar return null for a 404
Pass $info to Connection::decodeResponse()
Merge branch 'master' of github.com:jasny/social