Add CocoaPods support and installation steps

[qasim]

Because of this project's interesting dependencies, I don't have an Airstream.podspec yet, nor can this build on its own with something like Carthage. You can, however, just drag this project into your own Xcode project and add it as an embedded binary.

The challenges to get this to work with CocoaPods:

• Build libshairplay.a and its dependencies alac.a + crypto.a
• Build Airstream.framework

[qasim]

I've fixed up the project so that it can build with Carthage, but CocoaPods is still not available.