Create a loginHandler
FIX: Set the created DatabaseConnection to the instance

Otherwise it was created every time "getInstance()" was called.
Add a namedQuery to find Users by there Usernames
Add the login logic to the login handler

The LoginHandler requests the user with the corrisponding name from the
database and checks if the password is correct.
If everything is fine it moves on to the overview.
Otherwise it throws a LoginException.
Use the new login logic in the LoginPanel
Merge branch 'login' into 'logic'

The login functionallty itself works fine, but there are some troubles
with JPA which have to be fixed before it really works.
(And ofcourse there needs to be a user in the database)