Enable NuGet packaging of Py/WinRT

[devhawk]

• Define XLANG_VERSION_STRING macro for pywinrt and cppxlang via target_compile_definitions
• emit XLANG_VERSION_STRING in generated python files
• remove reliance on MIDLRT naming convention to detect property get/set and event add/remove methods
• add print_usage for command line help text
• move existing C++/WinRT package contents in /src/package to /src/package/cppwinrt
• add nuget package assets for Py/WinRT to /src/package/pywinrt
• change default build version to 0.0.0 (typically, build server explicitly passes in the build version to use)