Implement database/sql/driver.DriverContext

In order to fully instrument SQL operations (including new conns)
by wrapping this driver with something like
`github.com/luna-duclos/instrumentedsql`, this driver needs to
implement `database/sql/driver.DriverContext`.

Now it does.

conn.go

@@ -48,6 +48,15 @@ func (d *Driver) Open(name string) (driver.Conn, error) {
 	return Open(name)
 }
 
+// OpenConnector sets up a new Connector, ready to create connections. name is
+// a connection string. Most users should only use it through database/sql
+// package from the standard library.
+//
+// Implements `database/sql/driver.DriverContext`.
+func (d *Driver) OpenConnector(name string) (driver.Connector, error) {
+	return NewConnector(name)
+}
+
 func init() {
 	sql.Register("postgres", &Driver{})
 }

connector_test.go

@@ -65,3 +65,24 @@ func TestNewConnector_Driver(t *testing.T) {
 	}
 	txn.Rollback()
 }
+
+func TestNewConnector_DriverContext(t *testing.T) {
+	name := ""
+	d := &Driver{}
+	c, err := d.OpenConnector(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	db, err := c.Connect(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	// database/sql might not call our Open at all unless we do something with
+	// the connection
+	txn, err := db.(driver.ConnBeginTx).BeginTx(context.Background(), driver.TxOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	txn.Rollback()
+}