Bootstrap installation with a modern version of setuptools

Addresses #143.

Setuptools 0.6.10 is the oldest version with which we have tested Healpy, and is also the least common denominator present on Scientific Linux 6. If the user has setuptools >= 0.6.10, just take it. Otherwise, let `use_setuptools()` download its default, more recent version.

While we are at it, update to the latest copy of `ez_setup.py` from the Setuptools project.