allow dropping pg databases even if there are active connections

You gotta get tricky with postgres to get around there restriction
on dropping databases that have active connections.

See:

* http://stackoverflow.com/a/15729440
* http://www.postgresql.org/docs/9.2/static/functions-admin.html#FUNCTIONS-ADMIN-SIGNAL-TABLE
Merge pull request #34 from redding/kr-pgdrop

allow dropping pg databases even if there are active connections