Fix for the #198 fix

Another fix for #198. Also fixes #200, that's a very nice side effect. #200 was the number one reason colleagues stood near my desk with buildout questions :-)

From the changelog:

- Improved fix for #198: when bootstrapping with an extension, buildout was
  too strict on itself, resulting in an inability to upgrade or downgrade its
  own version.

- Setuptools must be at 3.3 or higher now. If you use the latest bootstrap
  from http://downloads.buildout.org/2/bootstrap.py you're all set.

- Installing *recipes* that themselves have dependencies used to fail with a
  VersionConflict if such a dependency was installed globally with a lower
  version. Buildout now ignores the version conflict in those cases and simply
  installs the correct version.