rename DiscId to Disc

DiscId is still there (inherits from Disc), but a deprecation warning is
printed on stderr.
Merge branch 'master' into track_class

This should merge some module-level read fixes

Conflicts:
	discid.py
create discid package / split module, re #25

This splits the discid module into multiple modules:

libdiscid.py loads libdiscid and extracts the constants
disc.py is the Disc class and related functions (read/put)
util.py has utility functsions (_encode/_decode)
move constant "docstrings" to __init__.py

This way sphinx can access them.
Either way sphinx thinks they are defined in the discid package,
and tries to extract the "docstring" from there.
Actually the "docstring" is just sphinx reading the source in that case.

Another option would be to access them as disid.libdiscid.* in sphinx,
but that wouldn't document them as being in the main package.
Merge branch 'package'

LIBDISCID_VERSION_STRING needed a manual split in modules.

Conflicts:
	discid/disc.py