Add Support for Swift Package Manager

[mattprowse]

See inline comments for explanations of some of the less obvious changes.

I tried getting this to work without having to move the resources into their own folder, but the Package.swift exclude list doesn't seem to apply to resources and and I got SPM errors that I couldn't work out how to resolve any other way.

SPM is also very particular about how the public headers are provided. I used an approach I've seen used in other Mac libraries which was to create a dedicated includes folder for SPM which contains symlinks to the relevant headers in their original locations.

I also added instructions to the README (and while I was there I added instructions for CocoaPods and Carthage too.