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
not a blocker, but... despite being a private method... `get_stats` does a lot of the logic so there should be tests to cover what it is doing, either indirectly (for example, not stubbing `get_stats` and calling `get :index`). Would be nice if the test coverage included this logic :grinning: