Add copyright

I am really enjoy this library, thanks you! I am going to make it a pod and push it to CocoaPods. 

The specification for CocoaPods mandatory requires a LICENSE, and a tag. I've setup that in my fork, it would be better the official repo could include a LICENSE as well. Once added, I will update the podspec so that others can install it directly from this official repo.

If you think the license type is not your favor, please choose the right one. Any type is ok.