Sample Universal Apps for Windows 8.1 / Windows Phone 8.1 Added

Sample application for Windows 8.1 and Windows Phone 8.1. Added. Issues
with executing both.  Phone version has issues with If
(ModeDetector.InUnitTestRunner()) in RxApp.cs, whilst Windows 8.1
version has issues with RxApp.MainThreadScheduler = new
WaitForDispatcherScheduler(() => CoreDispatcherScheduler.Current); in
Registrations.cs.