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.
Repository, really in this specific use case it doesn't matter, but its usually logical to keep up to date as you use it for `pod trunk push`