Add GitHub Basic Authentication via OAuth Tokens.

Hi Gary,

I'd like to get your feedback on the following proposal for a PR.

It adds the ability to set the GitHub credentials using Basic Authentication via OAuth Tokens, as documented [here](http://developer.github.com/v3/auth/) in the GitHub API docs.

This way, it's possible for an instance of GoDoc to have access and index private repos, something GoDoc normally cannot do because it would get 403/404 errors from GitHub API when trying to access private repos.