- Removed several parts to be installed by default, since some packages are
  not ported yet.

- Added support for Python 3.3.

- Replaced deprecated ``zope.interface.implements`` usage with equivalent
  ``zope.interface.implementer`` decorator.

- Dropped support for Python 2.4 and 2.5.

Overall, porting the package with all the doctests was not that
problematic.
Document some remaining tasks for Python 3 support.
Preparing release 3.0.0a1
Back to development: 3.0.0a2