Add test case for transaction rollback.  Currently fails with a nasty error.
Handle 'C' command complete message
Editorialize comments

Right now there is no obvious way to push command tags through
database/sql, so just drop it on the floor.  libpq does little more
than report it and transition the state machine to 'READY', and as of
now Go pq has no state machine model to speak of.

Signed-off-by: Daniel Farina <daniel@fdr.io>