Add Podspec

This adds the Podspec for Quick, it is working on both Artsy's [Eidolon](http://github.com/Artsy/Eidolon) and [Moya](http://github.com/ashfurrow/Moya). I based it of the only tag that exists right now.
This is the version of the podspec, or of the repository? If the latter, we should use [v0.2.0](https://github.com/Quick/Quick/releases/tag/v0.2.0)!