Activity feed

PT story: https://www.pivotaltracker.com/story/show/77003240

- added `public_activity` gem: https://github.com/pokonski/public_activity
- added `PublicActivity::Activity` model
- record `create/update` actions on:
  - `Document`,  `Project`, `Article` 
- displays activity feed on `/dashboard` 

##### See Demo at: http://wso-events.herokuapp.com/dashboard 
This is a first version of the feed. When we are done with the events epic we will add tracking of event actions and incorporate feeds on projects and individual users. Also, a better place to display this feed is on the landing page rather then the dashboard, but we'll need to wait for the redesign story to get started. (eg https://www.pivotaltracker.com/story/show/77633926)
![skarmavbild 2014-09-15 kl 06 18 02](https://cloud.githubusercontent.com/assets/5248170/4267365/73313d5c-3c9a-11e4-9e24-531f1554c7dd.png)