Add a podspec.

I use LlamaKit in an [open source app](https://github.com/artsy/eidolon) I work on. CocoaPods has swift support [still in a a PR](https://github.com/CocoaPods/CocoaPods/pull/2835), but it's nearly there. This adds CocoaPods support to LlamaKit and it would be awesome to have it in the master branch on your repo.