Added support for Windows 8 and 2012

Hi Chris,

As mentioned on a previous issue, I tested Windows 2012 and 8 with Oz and they both work. This patch add support for these versions and a small refactoring of Windows.py.

Also, you'll see that I increased the default timeout for Windows 6.x family, this is because during my tests I saw that, in the same environment, Windows 8 took around 8000 to be installed (while the others were installed within the previous timeout value of 6000).

Finally, the defaults xml for both versions are the same as you had previously for windows 7 and 2008.

That's all!

Cheers, Luis