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. The only changes to FixTk.py were to replace sys.prefix with sys.real_prefix at lines 49 and 52. 

Fixing this is really important because it affects matplotlib.

All nosetests pass and matplotlib and Tkinter work on python 2.6 and 2.7. Issue #93 referred to Python 2.6 so this would close it. A quick test importing tkinter and calling tkinter._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.

This looks pretty scary, and without any means of testing it, I worry it may be a source of problems. Is it really necessary? What is the likelihood of there being a symbolic link here? It seems to me that symlinks are rare on Windows, and while that doesn't mean we should completely fail to handle them, I'd prefer to add code for them based on a real use case rather than "just in case".

And also, why only on Vista? Doesn't the same issue of symlinks exist on XP?