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 fine, but I should really update the README. I feel like "BDD" is a loaded term.