BUG: the wrong executable was being found ...

... despite the right prefix being passed due to an embarrassing bug in
the code.
Merge pull request #94 from prabhuramachandran/bug_in_get_executable

BUG: the wrong executable was being found ...
BUG: Do not remove site-packages even if it's empty.

pip and easy_install do not work, when site-packages directory is not
present.  Uninstalling a lone packages that was installed using
enstaller removes the site-packages directory, causing other tools to
not work.
Merge pull request #96 from punchagan/keep-site-packages

BUG: Do not remove site-packages even if it's empty.