Search base class

I thought it may be good to make all the searching a bit more consistent by using a base class as we are writing the same code a lot of times.

The file: https://github.com/hahmed/octokit.net/blob/2551ce867e9c3ad3c98256a19566d110a8c6eeb9/Octokit/Models/Request/BaseSearchRequest.cs

I have added an example on how this can be implemented, with the SearchCodeRequest class.

I would love to hear your views @shiftkey and @alfhenrik

https://github.com/octokit/octokit.net/pull/290

(Thanks to both of you guys I learnt a lot from the searching api :thumbsup: )