Adds support for unread actions

Hi,

First of all, thanks for django-activity-stream, which fulfilled almost all my needs.

These commits add the ability for a user to have actions marked as read/unread.
The behaviour is governed by the TRACK_UNREAD_DEFAULT and AUTO_READ_DEFAULT settings.

Have been using it for a few weeks now (not in production though) without any problems.

Feel free to comment / make suggestions.

Thomas