mpg123 input plugin

This is a work in progress.

This patch adds an mpg123 input plugin.

1. This fixes #100
2. This fixes #3 (no seeking and incorrect time remain)
3. Loading the meta data of 4000 mp3 files takes 2 seconds instead of 20 with libmad
4. The code is much simpler

You can test it right now with ./configure; make; make install since mpg123 has the highest priority. I'll change this later because I only parse a subset of the meta data parsed by libmad. But in my case it gets all but one file right.