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: 
Note that there's no guarantee if or when ```__del__``` will be called (it's when the object is actually deleted by the reference counting, not just when it goes out of scope, and things like cyclical references can prevent it from happening at all)

If this was a long-lived server process then I'd suggest using ```.close()``` but since it's a unit tests and you know the interpreter is going to stop it's not a big deal. See http://initd.org/psycopg/docs/usage.html#transactions-control for instance, especially the note about idle transactions (not something you want a lot of on a busy database since there is a limit to the number of sessions on the database side)