Handle 12.00 in XBuild's SolutionParser.

This doesn't fix support for VS 2013 projects, but it's a start...
[xbuild] Build & install MSBuild 12.0 assemblies
[xbuild] Install data files for xbuild 12
[xbuild] Make xbuild script point to xbuild 12
[xbuild] Update printed version/copyright
[xbuild] MSBuild 12 toolset should be available for XBUILD_12
[xbuild] Fix 12.0 tasks
[xbuild] Fix 12.0 toolset location and public API