Conditional requests

Since making conditional request does not count against rate limit, it would be great to support it: http://developer.github.com/v3/#conditional-requests

I think, It would be easy to do with additional faraday middleware. This middleware would add information from response version headers (Etag, Last-Modified) to resulting Hashie::Mash. Also it would convert appropriate request params (etag, last-modified) to If-None-Match, If-Modified-Since headers.

What do you think @pengwynn?