Fix Tcl inside a virtualenv on Windows (issue #93)

See issue #93 for details and alternative solutions. One suggestion was to bring code from Lib\lib-tk\FixTk.py into site.py, but leaving site.py alone and copying a minimally modified FixTk.py to the Lib directory when the virtualenv is created seems cleaner to me.

Fixing this is really important because it affects matplotlib.

All nosetests pass on python 2.6 and 2.7 and matplotlib works, as well as Tkinter. Issue #93 referred to Python 2.6 so this would close it. A quick test importing tkinter and calling t._test() in a Python 3.4 virtualenv shows the problem is unresolved there, but it probably doesn't matter anyway since Python 3.3 and venv.