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 could
not reliably track different versions of WinJS. This change
leverages the existing string replace grunt task and exposes
WinJS.Utilities._version.
Merge pull request #790 from seanzer/telemetry1

Add a private global property to track version