Total Github Commit Count for Projects

Pivotal Story: https://www.pivotaltracker.com/story/show/80236842

* Add field in projects table to store total `commit_count` for project
* Using Nokogiri to scrape the total commit count of the projects github repo
* Displaying commit count on Index page

Screenshot:
![gcc](https://cloud.githubusercontent.com/assets/1736001/4616139/93632b5a-52f0-11e4-8ad2-2c35fe7cc1f0.jpg)