Normalize TOC "DATA" paths to prevent multiple inclusion on Windows

This is implements the patch suggested on
http://www.pyinstaller.org/ticket/783

This should also fix a similar problem on OS X when
case-insensitive HFS+ is used.

We are internally using a version of PyInstaller with this
patch since November 2013 on a couple of production
projects targeting both Windows and OS X. We are not
aware of any ill effects yet.