Restructure module

* "Hide" the extension as libdiscid._discid.
* For compatibility libdiscid.discid provides everything that has been available
  from there before.
* Move DiscError to libdiscid.exceptions and make it available as
  libdiscid.DiscError.
* Move most of the API documentation back to libdiscid/__init__.py.
Store the device

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Improve the documentation

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Prepare changelog and versions for 0.3

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Improve test coverage

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Fix test

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Add a note about the eggs

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Fix requires passed to setup

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>