Add cocoapods spec file

Added a CocoaPods (http://cocoapods.org/) spec file for the category.  CocoaPods requires both a tag and a license file, so in order for this to be submitted there needs to be a "v1.0.0" tag available in the repo.

I was going to make a pod spec myself for this category (which I'm using in my own code) and then found your repo, so figured I should just do a PR and add the spec to yours.