beets: enable gstreamer support

So this was suggested as [long ago as October, 2015](https://github.com/NixOS/nixpkgs/issues/10376#issuecomment-147734898).

Despite being fairly ignorant of the nix Python support, I decided
that I really wanted this; this change brings in what I believe are
the necessary components---I have, at least, successfully run `beet
replaygain` and `beet bpd`---but it may not do it in the best way; I'm
happy to consider input on that front.

I can at least state that all three changes are necessary---leave any
one of them out and gstreamer support doesn't work.