Isolate about controller

Main idea was to isolate the About controller so testing is easier to read.

I've ended up refacoring it a bit, so final outcome is:

- An adapter for the external Github SDK. This way we have better control and we can change it in a easier way.
- A service for silex. So we can re-use and share in other controllers. A plus is that was easier to mock :yum: 
- Reversed the contributors list. This was due because I noticed that major contributors where at the end of the list. And I don't thing is fair for them :no_good: