Fix for changes to virtualenv defaults in 1.7.0
Fix for changes to virtualenv defaults in 1.7.0

In virtualenv 1.7.0 --no-site=-packages = True became the default, and
a new cmdline switch was adding to set opposite.

This patch always sets the xx-site-packages options , and tests for the
existence of the new options and sets that as well.

nikolas/paver,thedrow/paver,phargogh/paver,gregorynicholas/paver,cecedille1/paver,cecedille1/paver,gregorynicholas/paver