Adds WebAuthBroker native authentication UI on Windows8/8.1/WP8.1 platforms

When Web SDK is injected into Phonegap/Cordova, WebAuthentication broker object is available and can be used to provide more native authenticate UX.

This PR uses code from WinJS login implementation: (https://github.com/Azure/azure-mobile-services/blob/master/sdk/Javascript/src/Platforms/Platform.WinJS.js#L131)