crontabberise drop_old_partitions; bug 1014128; attempt #2

This addresses bug 1014128. Again. (Previous attempt at rebasing was a horrible disaster. :unamused:) 

`?r` @peterbe and/or @rhelmer :revolving_hearts: 
http://initd.org/psycopg/docs/usage.html#transactions-control says ```close``` is an implicit rollback, but I am fine w/ making it explicit.