support EmptyQueryResponse message type

If the user wants to issue ```conn.Execute("--;")``` or similar noop to ensure a database connection exists, they would get ```Received unknown message type: I```.  Which, according to [the docs](http://www.postgresql.org/docs/9.1/static/protocol-message-formats.html) is a special message type that "substitutes for CommandComplete".   We didn't parse this message type, but it seems trivial enough to add.