Build files for VS2012 + relevant .gitignore changes.

This is it : the build files for Visual Studio 2012, tested on all four combinations of debug / release and x86 / x64, on Windows 7 with a fresh install of Visual Studio 2012 Ultimate. Fix for issue https://github.com/mmp/pbrt-v2/issues/13.

I took the liberty to add relevant ignores to the .gitignore file (in a separate commit): the intellisense files / folders, the user property file, and the various build folders which differ from the unix ones. All of these are generated automatically by Visual Studio. I could have changed the build folders in the properties of the projects, but since I imported the project files from the VS2010 ones, I went for the most conservative route.

(And of course, thanks for the amazing book and software !)