Dashboard stats

PT story: https://www.pivotaltracker.com/story/show/64726524
Screenshot: 
![image](https://cloud.githubusercontent.com/assets/1007981/4091279/dcd318aa-2f86-11e4-9b60-8ed0f335440e.png)


Changes:
• dashboard controller
• statistics concern
• dashboard view
• link at bottom of global footer
AFAIK, this is not the correct use of a concern. A concern is used for sharing code. In the case of a controller concern, it is used to share code between similar controllers.

I think this should be changed to a non-AR model called `Statistics` which exposes methods like articles, projects, members, documents, etc.