Add .podspec

There's a .podspec for ParseKit on CocoaPods, but it's not included in the official repo and that makes it a bit troublesome to try and fix some outstanding issues in the kit along with a CocoaPods-enabled app. Adding the .podspec file to the root of the repo solves the problem, so that you can always say in your Podfile:

pod 'ParseKit', path: '/some/path/on/your/local/filesystem/where/the/repo/is/checked/out'

Yours fork seems to be the most maintained on GitHub currently so I've decided to fork it and add some essential patches. ;)