Base class for searching GitHub

http://developer.github.com/v3/search/#search

We are repeating a lot of logic inside the searches, hopefully this class will allow us to simply our searching.

I will close https://github.com/hahmed/octokit.net/pull/1 so we can keep our discussions on the octokit repo (if anyone else has any input).

How do you want this to be merged?

I have created the base class, so we can now iterate on it till it's right...

Once this is merged - we can open a PR for every search to inherit from base class? @alfhenrik can then use it for his PR https://github.com/octokit/octokit.net/pull/290 and I can also use it for https://github.com/octokit/octokit.net/pull/289

cc:// @shiftkey  @alfhenrik