Two new QP filters: create_date and edit_date

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

[LFW] Reviewed and added a single missing 'use' line; rewrote commit
message; release note.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>
Allow nesting of create_date and edit_date filters

As gently suggested by tsbere, this commit allows nesting of said
filters.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>