Implement large object support

This patch implements large object support as described in
http://www.postgresql.org/docs/current/static/largeobjects.html

conn.go

@@ -50,6 +50,7 @@ type Conn struct {
 	causeOfDeath       error
 	logger             Logger
 	mr                 msgReader
+	fp                 *fastpath
 }
 
 type PreparedStatement struct {