Add a private global variable to track version

This variable is useful for sites which measure usage of various JS
libraries. WinJS did not have a globally exposed variable, so sites know
if one uses WinJS, but not the version with accuracy. This change
leverages the existing string replace grunt task and exposes
WinJS.Utilities._version
Why not put this string into _BaseUtils directly? Is something that only depends on _BaseCoreUtils going to need this?