windows: better msbuild version/path parsing and query correct registry on 64-bit

Tested on VS2010 Express on WinXP 32-bit and Win7 64-bit (32-bit and 64-bit node v0.10.x), and VS2012 Express on Win8 64-bit (32-bit and 64-bit node v0.10.x).

This fixes at least some build issues I was having when I did a fresh install of both Win8 and VS2012 Express and was using the regular command prompt.