Mostly bug fixes and updates to the WP8 sample app

- Crashing bug in the FriendPicker control
- Exception when changing SelectionMode
- Updated default value for DisplayOrder in FriendPicker for WP8 (to match Windows 8)
- Fixed incorrect behavior and improved user experience when using the FriendPicker's jump list 
- Updated the sample application for Windows Phone 8 to align it with the tutorials (and the Windows 8 app).