New package for ALSA volume tray applet, pnmixer

First contribution to nixpkgs, which is pretty exciting. Then again, this was a really smooth and easy process, and I'm already using the package locally, so I'm *somewhat* confident I haven't buggered everything.

https://github.com/nicklan/pnmixer

You'll notice there are some compilation options that are not exposed by my package - using gtk2, for example, or disabling the usage of libnotify. I considered adding support for these, but it probably makes more sense to wait for someone else to scratch their own itch, thus proving there's anyone who cares. Plus, the more complexity in a single PR, the more likely for things to be subtly wrong - right now, it's nice and simple as it is.