GitHub integration

Implemented the ability for users to create timelines by importing issues, commits, and pull requests from Github repositories.

Github interaction is accomplished using the Octokit gem.

The new Repository controller exposes 2 public actions:
1) #new will return a list of the current user's Github repositories, assuming the user has linked a Github account. If no such account has been linked, an empty list will be returned.
2) #create accepts a name (for a new timeline) and an array of repository names. A timeline will be created for each specified repository. If multiple timelines are created, they will be nested under a single parent.