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.

> Also, if this is a copy/paste and edit of the core FixTk.py, won't any changes core Python makes in later versions be lost?

That's actually already the case for virtualenv's `site.py` that it has vendored / modified - it's missing functions that were introduced in 2.7 See https://github.com/pypa/virtualenv/issues/355

But since this is for 2.X only and we already know there's no 2.8, I can't imagine that is a worry for this particular file