Sync'd podspec with podspec from Cocoapods Specs.

The podspec in the CocoaPods Spec repo is not maintained by @ccgus (you mentioned before, that you do not use cocoapods). After merging #179, I noticed that the podspec in the CocoaPods was not the same as the spec the one in the project.

This PR contains the same [podspec that has been submitted to the CocoaPods/Spec repo](https://github.com/CocoaPods/Specs/pull/4874)