Support followed cookbook activity feed for users

:fork_and_knife: Users should be able to subscribe to a feed that contains all of the cookbook activity for cookbooks that they follow.

![screen shot 2014-07-24 at 2 54 38 pm](https://cloud.githubusercontent.com/assets/316507/3693323/e4b87656-1365-11e4-8089-54740f0276d7.png)