Two new QP filters: create_date and edit_date
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These filter on the fields of the same name in biblio.record_entry.

e.g.
  * create_date(,2013-02-01)  =>  records created before 2013-02-01
  * create_date(2013-02-01)   =>  records created since 2013-02-01
  * create_date(2013-02-01,2013-02-28)  =>  records created in Feb 2013
  * create_date(yesterday)    =>  records since created yesterday