Add podspec file

As of [0.36.0.beta.1](073888038b01bd1dd9c99537313ff1c9b44b86ec), CocoaPods supports Frameworks and Swift source files

I tested it with a basic project with the local podspec file. I wasn't sure if I needed to include the `.h` files as well to `s.source_files`. Let me know if I should make that change.