add controller method to export current_user

[rosabraatz]

To allow the rating-widget to show information about the current_user, we need a new controller method. It should return profile_picture, first_name, last_name, user_id.

[MrSerth]

We might need to set the HTTP Access-Control-Allow-Credentials in order to allow our script to send the session cookie to this route. Have a look at this StackOverflow thread: http://stackoverflow.com/questions/2870371/why-is-jquerys-ajax-method-not-sending-my-session-cookie