Add fetching commits from a repository

Allows the user to fetch commits from a specific branch or SHA.

I am not satisfied with the class name `OCTGitCommit` but `OCTCommit` was already taken.

`OCTCommit` differs from `OCTGitCommit` as the api end point is a different.

This uses: http://developer.github.com/v3/repos/commits/#list-commits-on-a-repository