Add cocoapods spec

Hey foursquare people!

I love the lightweight design of FSNetworking and we're looking to use it on a new iOS project at HubSpot. We use CocoaPods for dependency management -- could you add this podspec to your repo so people can pull it in with CocoaPods instead of manually inserting the files?

(Right now, it links to my fork of the repo, but once the Podspec is in yours, I imagine we'd want to move it over.)