Infodrom Oldenburg

This patch adds several bugfixes and improvements to the nice calendar by Oliver Bryant:

• fix leap year calculation
• allow the week to start with arbitrary day (0=sun, 1=mon etc.)
• add close button
• allow past dates
• allow other delimiters besides '/'
• add path to images
• make everything configurable via »calendar(options)«
• use the current month and year as default for all empty/broken input
• add »popcalendar(id)« function for image buttons