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.

Is it *meant* to be for 2.x only? If so, the file should only be written on 2.x. And if it's not, the point remains for the equivalent file on 3.x - copying the file has risks. (OTOH, it may be that the 3.x version fixes this issue as it presumably supports venv...)

But yeah, it's a relatively minor point.