Added more attributes in Config to give more customization of the Calendar

This is a really small contribution to make the Calendar have configuration option I needed.

I just added more attributes in Config object. They help to make the Calendar 'i18n' compatible, whether it displays weekends or not, and the first day of a week.
Added Javadoc on the accessor of these new attributes.

This code does not affect Wicket version, it can then be deployable in older release of the project.