Add faust, the functional audio stream programming language

This package has just the bare Faust compiler, which will compile a faust.dsp file to a c++ file.
A full Faust development environment includes a group of scripts that compile that c++ file to a binary.
As there are around 30 of these, with lot's of dependencies, which are all optional, it was decided on IRC that these would be made into a separate package. Work has been done on that as well.

PS: this is my first pull-request ever, plus I'm new to NixOS. My apologies for any errors in form or content.