add DiscError

so we can catch it independently from other IOErrors.

The same Error was used in python-musicbrainz2.
Use the LGPL3 for the module

This python module "discid" is under the GNU Lesser General Public
License.
No license restrictions for examples

The examples are not restricted by the LGPL.
Everyone is free to use, redistribute and/or modify them.
add README

This includes a short summary, usage and names the license.