Make ignore-site-packages default (if possible), add allow-site-packages

Hi,

This is basically a refinement of pull request #135, with the following behavior:

* site packages disabled be default
    * virtualenv will break without 'site packages', so nothing is done there. 
* allow-site-packages allows importing of site packages

I used the 'site' modules site package list instead of the just checking for 'site-packages', since most distributions register their site packages this way and the naming conventions 'site-packages' is unfortunately not standard.

This fixes the distribute import error on debian, Ubuntu, and OSX distributions. I haven't seen it fail on any yet.

I also took @tseaver's modification of the ez_setup to the new url as well, although that seems to bump up the requirement of setuptools up to 0.9.6. I prefer it, but I'm not sure if that's desired.