Add CocoaPods specification.

A podspec has been in the Specs for some time now, managed by a third party. It would be awesome though if UA would do that instead.

The differences between the current entry in the Specs and this one is that I split the UI and Core. In my project I don't use the UI and I don't want to clog by build process and .app with useless code and XIBs. Using `pod 'UrbanAirship-iOS-SDK/Core'` now only puts the core (non /UI) in the project.

The podspec I added is for 3.0.4, because the project I have that uses this does not use 4.0.0 (yet), so I can't test that. It probably only needs an update to the version numbers.

You might want to rename the Podspec and the pod (find&replace of UrbanAirship-iOS-SDK) to something like UAUrbanAirship.
Please let me know, and I will change it :)