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
I would suggest using a separate class/modules with exposed methods, something like `Statistics.for(:document)` instead of making this a controller specific function so we can use this beyond controllers.