Need to check if tests are complete/sufficient, and test with multiple Pythons
and on Windows.

Changed zc.recipe.egg and the easy_install.scripts function into only adding
-S to scripts.

New package z3c.recipe.scripts and the easy_install.generate_scripts now
contains the more robust interpreter generation and the ability to
include site-packages.

Pertinent tests from the previous approach (from 3+ months ago) are
ported.  In the course of this port I discovered that big changes like
the ones in this checkin were needed.