Custom OAuth2

Hey @atmos, still playing around with the code.

So, I started fiddling with the OAuth and thought that it is quite simple to do it "by hand" without relying on the oauth2 gem.

Not sure if you like a custom rolled solution, so feel free to dump this.

Anyways, I also changed the code to only use octokit to communicate with the API in another branch on top of this one. See the commits [here](https://github.com/fphilipe/warden-github/compare/custom-oauth...fphilipe:all-api-thru-octokit). I'm not sending a pull request for that one right now because I wanted to know if you think this branch makes any sense. If not, I would rebase the all-api-thru-octokit branch on top of master and send a pull request for that.

The nice thing about both branches is that they reduce the dependencies down to warden and octokit.

Tell me what you think! Have a nice weekend.