[Librefm-discuss] Using Composer

[Rob Myers]

I've made a version of the code that uses composer for dependency
management:

https://git.gnu.io/gnu/gnu-fm/tree/develop

gnu-fm/nixtape and gnu-fm/gnukebox have separate composer.json files. I
did originally make it so they shared a single composer.json in gnu-fm/,
but I split them out so that separate installation (as per the readme)
is easier. Feedback on this approach is welcomed.

Bug reports and feedback generally on these modifications also
gratefully received.

I'll merge this onto master after review.

- Rob.