- Added a way to authenticate against AMS.

- Added a way to authenticate against AMS.
   - Added AppSettings
       * EnableMobileServiceAuth
       * MobileServiceAuthenticationProvider
   - Added a AzureMobileServiceAuthenticator ServiceLocator that is implemented on WP8 and Win8.
   - Added a new View[Model] called InitView[Model] that will be called first when WP8 or Win8 is launched.
     - Authentication happens here to prevent loading of data before successful authentication.
- Changed the namespace of ItemsShowcaseView from XPlatformCloudKit to XPlatformCloudKit.Views.
  This prevented base.ShowViewModel() from finding the View for ItemsShowcaseViewModel.
- Phone8::App.xaml.cs - Changed Application_Launching() to hook the RootFrame_Navigating event and
  initial the ServiceLocator and IOC stuff.  This resolved raising condition observed when the service is trying
  to authenticate against AMS.  Basically, authentication must start only after Mbx and ServiceLocator are both
  initialized.
- Refactored AzureMobileService class so that the AzureMobileServiceAuthenticator can reuse AMS's MobileService instance.