Activity stream enhancements.

This is some work I've done to improve the activity stream in the downstream Geonode project I maintain.  

   * Moves social_signals.py into a new geonode.social app where we could store additional social functionality.
   * Improves the styling of the activity stream.
   * Adds a "recent-activity" view that shows all recent public activity.

I know this violates the feature freeze so we can hold off until 2.1 if preferred.  Please add code comments if there are pieces you would like for me to refactor.

![Activity Stream](http://i.snag.gy/nyPOS.jpg "Activity Stream")