Added SetParity (still missing windows implementation)

serial.go

@@ -17,8 +17,21 @@ type SerialPort interface {
 
 	// Set port speed
 	SetSpeed(baudrate int) error
+
+	// Set port parity
+	SetParity(parity Parity) error
 }
 
+type Parity int
+
+const (
+	PARITY_NONE Parity = iota
+	PARITY_ODD
+	PARITY_EVEN
+	PARITY_MARK
+	PARITY_SPACE
+)
+
 // Platform independent error type for serial ports
 type SerialPortError struct {
 	err  string