Basic Github Integration

I highly recommend a complete db drop, create, migrate, seed cycle after merging/pulling down this pull request. I changed and implemented so much crap with this branch that I honestly don't remember it all, but you can follow it through the commit messages below as they're usually pretty good about being descriptive of what was done. 

Some highlights:
* Users can link their user account with their github account
* Users can login to app using their github account via omniauth once linked
* Break out github info into separate db table for github_profiles
* Implement handling of webhooks from github for pull requests and issues
* Creates new assignments from a new github issue being opened on a repo that is added to a cohort
* Creates a new answer from a pull request that is opened on a repo that is added to a cohort if it uses id tag
* Updates breadcrumbs to be more dynamically built
* Cleans up lots of code throughout, but also adds quit a bit of smelly code as well

Some of the code isn't the cleanest and is in serious need of cleaning. Especially around the views.
Oh! I do remember now. The gravatar image.
I wonder if I would be able to use something similar to this block for the discussion board avatars then.