Fixed serializable issue for bills
Interaction with books/reservs/bills fully works
BalanceController occupancyRate & financialBalance woerks
Discount working
BalanceController function occupancyRate bug fix
Updated AccountController, template "buchung"

- now you can create campers, groups and bookings with the GUI
Integrated search and error messages

Note: search does not work at the moment because of a bug in
BookingController.isBookable()
Added catching for date parsing exceptions
Improvements for UI "buchung"