Added a basic podspec for DCTAuth

I'm trying to ensure every third party library I use in my apps from now on is using CocoaPods rather than git submodules.

For those that don't support it yet, I'm submitting working Podspec files in the hope that they'll be merged into master and submitted to the [CocoaPods Spec repo](http://github.com/CocoaPods/Specs) for use by everyone.