Adds option to pass ProductKey paramater for VisualStudio 2013 Pro, Prem, and Ultimate

This fixes Issue #15 where there wasn't an option to pass a product key through to Chocolatey when installing Visual Studio 2013.

Affects Professional, Premium, and Ultimate. Ran test installs on all 3 to verify it's working, but feel free to test yourself. I did some significant refactoring in order to parse out different arguments passed by Chocolatey and ended up extracting a common file for each of the installer scripts to use. They all basically do the same thing now except call a different .exe

Let me know if you have any questions on this.