Github Commit Count

https://www.pivotaltracker.com/story/show/64891298

* Displays a user's contribution to different projects on profile page
* Introduces a `GithubCommitsJob` which fetches contribution statistics from the Github API using the octokit gem through unauthenticated requests.
* Introduces a rake task which provides a interface to run the `GithubCommitsJob` through `rake fetch_github_commits`.
* The task is supposed to be run every 24 hours through the Heroku Scheduler.

![gcc](https://cloud.githubusercontent.com/assets/1736001/3969985/de181a58-27c5-11e4-9b88-38aecbd46bb9.png)

I paired with [Gordon](http://agileventures.org/users/gordon-duquesnay) on the backend part while I did the frontend solo.

Using the relationship introduced in this PR, we can get a list of active users for a project using a `has_many :through` relationship: 

`has_many :active_users, through: :commit_counts, source: :user`

This allows us to find a list of users who are contributors to the project using: `project.active_users`.