VSIX tooling changes to support Visual Studio 2013.
Use the target platorm version in the VS props file as well.
Update comments and package flavor error message.
More changes to support MSVC 18.
Use the VisualStudioVersion variable instead of _MSC_VER.
More corrects to VS 2013 integration.
Environment variable names in nmake must be in all uppercase.
VSIX tooling changes to support Visual Studio 2013.