Add support for Repositories Contents API

[EtienneDepaulis]

Gets the contents of a file or directory in a repository

Doc: https://developer.github.com/v3/repos/contents/#get-contents

Do you want to be able to manage options like ref or when it's a directory (we get a listing).
For the last one, checking the other endpoints, it seems the philosophy is to be only an API wraper.
What about the the ref option then ?

[coveralls]

Coverage decreased (-0.3%) to 91.391% when pulling 8d4d7b3 on EtienneDepaulis:repositories_content into 56c1f75 on edgurgel:master.

[edgurgel]

Up to you really! I think it's fine for now and we can add later as an optional argument?

[edgurgel]

Thanks for the PR! I will release a new version soon 🎉