Bugfix #273

I fix bug #273 (printing no longer working).  There was a problem while
retrieving the User.helpdesks collection when the Hibernate session is
already closed.  Because we only need this collection to fullfil the m:n
relation structure I implement a domain model class for the relation
table and change the way the users are added to the helpdesk.
Use embedded addresses

I implement a class Address and declare it as embedded property in
organizations, persons, and invoicing transactions.  Furthermore, I
adapt the views and the test cases.

Additionally, I implement a new menu item at addresses to clear the
address input fields.