Add hooks API

Hi there!

I have added the github [hooks API](https://developer.github.com/v3/repos/hooks/) and PubSubHubbub.

I haven't put all needed tests, as I don't know how to tests them actually (I'm using Resharper that doesn't recognize Xunit... ah, nevermind, I remember that I probably need to install a plugin... ok...)

Probably not mergeable as-is, but at least you get a grasp of the work.


As this is documented under `Repos` -> `Hooks`, can this be moved to a property on `IRepositoryHooksClient`? 

This will also keep the top level API less cluttered.