Custom backend OAuth2 example based on AFNetworking

This pull request is based on the previous one I've made, just because it was easier to add the `AFNetworking` and the `AFOAuth2Client` dependencies to the example project.

I only made the base of an example and it's clearly not complete. The goal is to show you how I see it, to check if I can "read in your mind"... figure of speech of course :)
I mean, check if the track I'm taking is good for you.

Before coding those few lines I was thinking about the great [speech](https://vimeo.com/47459338) Mattt gave at NYC, and how he said it's hard but essential to keep an open source project simple and avoid taking decisions to maximize the flexibility.
Well, I have no pretention to write as beautiful lines as he does but this little digression simply helps me to explain why I implemented the OAuth2 login as an exemple and not a core method of `SDLoginViewController`.

Am I on the right path ?