Add support for Swift Package Manager

[klaas]

Would be great if you accept this pull request.

I'm using CupertinoJWT for Apple MusicKit and it works fine!

[ethanhuang13]

Thanks, but CupertinoJWT supports not only macOS but iOS, tvOS and watchOS. You may need to update the file.

I also plan to make CupertinoJWT to required Xcode 10 to natively import CommonCrypto.

[DominicHolmes]

Any update on this? Would love to use this in a vapor project, but we require SPM