Create MessageBox dialog service; Handle app exit event and display message

- Created IMessageDialogService interface
- Implemented IMessageDialogService in MessageDialogService class using WP Toolkit CustomMessageBox
- Moved some comments to wiki  
- Added exit confirmation dialog

Later there will be issue with services to fix. After pulling Mikołaj's and Bartek's requests, we should move services created by them to MyGuide.Services namespace (folder) and reflector all affected code.