A saner way to bootstrap setuptools

This is a fix for #146 and the regression introduced in https://github.com/buildout/buildout/commit/9bcc9042b5ff40b3a3fb24d1b4fce7d95549e51d 

With this patch bootstrap makes a best effort to use existing setuptools if installed or available in find links.

Either respects the --allow-site-packages options and use setuptools there if installed.
Or honor the --find-links if there is an ez_setup.py available there.
Or as a last resort fetch and use the remote ez_setup.py