Terminate connections before dropping the database in postgres.

Postgres will refuse to drop a database that has active connections.
If you don't call `Close` in the right order in your tests, you can be
left with hard-to-debug, lingering databases. We can instead force Postgres
to terminate these open connections.
Give up on mysql
Merge pull request #4 from metcalf/master

Terminate connections before dropping the database in postgres.