Use setuptools' setup() if it is available

This permits users to use 'setup.py develop', which is something that has come up on the BioPython mailing list:

http://comments.gmane.org/gmane.comp.python.bio.general/7168

Since it depends on setuptools in this optional way, falling back to distutils.core, it seems safe to me to add this dependency.

I'm happy to discuss further.