Fixed indentation and Javadoc comments per Julien Dramaix's suggestions.
https://github.com/jDramaix/gwtchosen/pull/101
Added a convenience addStyledItem method.

Per Julien Dramaix's suggestion, added a convenience method,
public void addStyledItem(String label, String value, String className).
(see https://github.com/jDramaix/gwtchosen/pull/101 for details)