Audio converter rewrite and multichannel support

by Juho Vähä-Herttua for XMMS2 - X-platform Music Multiplexing System 2

Currently the XMMS2 converter is a large chunk of python generated code doing sample format conversion, linear interpolation and channel mixing. There has been suggestions to use libsamplerate, which is probably the best audio conversion library around, but its GPLv2 license is not suitable for the project. Goal is to add similar functionality with more liberal license, most likely something like LGPLv2+. At least the functionality of the current converter plus multichannel should be available.