In most U.S. libraries, if a borrower returns an item after the library has closed
on Monday, but before it opens  on Tuesday,  the  item is treated  as  though
it was returned Monday.   The item is usually placed into a  'drop box',
which is available when the library is closed.  This is coded and tested, but needs to be merged.
The accountlines table will store the last increment of an overdue fine so it can be
removed when returning an item with 'dropbox mode' enabled.

I do have some significant patches to Overdues.pm in addition to the dropbox patch;
I have encountered some problems with holidays calendar corrections on fines calculations.
Will submit these patches by late Thursday.