fix LIBDISCID_VERSION_STRING docstring for sphinx

Otherwise sphinx can't find and display the docstring.
use track objects, rather than multiple lists

This implicitely closes #24.
It would get really confusing to have a track object 0,
although one technically could also store lead-in information and
similar there.
fix tests for new Track class
Merge branch 'track_class'

see pull-request #27