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.


My psychic debugger here thinks you were adding these by hand.

We have a magic script to do this: just run `.\build FixProjects` from the root of the project and it'll take care of this (if you've got it all added to the `Octokit.csproj` file.