600 Baud comm. support, unsupported baudrate notification

- Added a 600 Baud line in DELAY_TABLE, for each CPU frequency.
(successfully tested for 16 MHz)

- begin(long speed) now returns a bool indicating if speed has been
found in DELAY_TABLE (if not, the method returns false immediately)

libraries/SoftwareSerial/SoftwareSerial.h

@@ -82,7 +82,7 @@ public:
   // public methods
   SoftwareSerial(uint8_t receivePin, uint8_t transmitPin, bool inverse_logic = false);
   ~SoftwareSerial();
-  void begin(long speed);
+  bool begin(long speed);
   bool listen();
   void end();
   bool isListening() { return this == active_object; }