Modifications to App and LoginController, see full message.

Changes:

- Add dependency on 'org.controlsfx:openjfx-dialogs:1.0.2'. This is required
  until JDK 8u40 (March 2015).
- Change MySQL Connector version from 5.1.32 to 5.1.34.
- Categorize documentation of App and LoginController classes.
- Move preference keys from App to LoginController.
- Change access specifiers in App and LoginController.
- Don't pass loggers around, create them in each class.
- Remove dead code.
- Update documentation.
- Update logging.
- Don't give LoginController instances access to the App instance. Pass
  preferences and the connection hook explicitly instead.
- Center the main window on creation.
- Add exception display functionality (App#unhandledExceptionHook). See #43.
- Use the new exception dialog in scene loading and in the connection hook.