Add Serial1-3.room()

http://forum.pjrc.com/threads/26064-Large-transfers-over-serial-on-Teensy-3-1
http://forum.arduino.cc/index.php/topic,165574.0.html

teensy3/HardwareSerial.h

 void serial_putchar(uint32_t c);
 void serial_write(const void *buf, unsigned int count);
 void serial_flush(void);
+int serial_room(void);
 int serial_available(void);
 int serial_getchar(void);
 int serial_peek(void);
 void serial2_putchar(uint32_t c);
 void serial2_write(const void *buf, unsigned int count);
 void serial2_flush(void);
+int serial2_room(void);
 int serial2_available(void);
 int serial2_getchar(void);
 int serial2_peek(void);
 void serial3_putchar(uint32_t c);
 void serial3_write(const void *buf, unsigned int count);
 void serial3_flush(void);
+int serial3_room(void);
 int serial3_available(void);
 int serial3_getchar(void);
 int serial3_peek(void);
 	virtual int read(void)          { return serial_getchar(); }
 	virtual void flush(void)        { serial_flush(); }
 	virtual void clear(void)	{ serial_clear(); }
+	virtual int room(void)		{ return serial_room(); }
 	virtual size_t write(uint8_t c) { serial_putchar(c); return 1; }
 	virtual size_t write(unsigned long n)   { return write((uint8_t)n); }
 	virtual size_t write(long n)            { return write((uint8_t)n); }
 	virtual int read(void)          { return serial2_getchar(); }
 	virtual void flush(void)        { serial2_flush(); }
 	virtual void clear(void)	{ serial2_clear(); }
+	virtual int room(void)		{ return serial2_room(); }
 	virtual size_t write(uint8_t c) { serial2_putchar(c); return 1; }
 	virtual size_t write(unsigned long n)   { return write((uint8_t)n); }
 	virtual size_t write(long n)            { return write((uint8_t)n); }
 	virtual int read(void)          { return serial3_getchar(); }
 	virtual void flush(void)        { serial3_flush(); }
 	virtual void clear(void)	{ serial3_clear(); }
+	virtual int room(void)		{ return serial3_room(); }
 	virtual size_t write(uint8_t c) { serial3_putchar(c); return 1; }
 	virtual size_t write(unsigned long n)   { return write((uint8_t)n); }
 	virtual size_t write(long n)            { return write((uint8_t)n); }

teensy3/serial1.c

 	while (transmitting) yield(); // wait
 }
 
+int serial_room(void)
+{
+	uint32_t head, tail;
+
+	head = tx_buffer_head;
+	tail = tx_buffer_tail;
+	if (head >= tail) return TX_BUFFER_SIZE - head + tail;
+	return tail - head;
+}
+
 int serial_available(void)
 {
 	uint32_t head, tail;

teensy3/serial2.c

 	while (transmitting) yield(); // wait
 }
 
+int serial2_room(void)
+{
+	uint32_t head, tail;
+
+	head = tx_buffer_head;
+	tail = tx_buffer_tail;
+	if (head >= tail) return TX_BUFFER_SIZE - head + tail;
+	return tail - head;
+}
+
 int serial2_available(void)
 {
 	uint32_t head, tail;

teensy3/serial3.c

 	while (transmitting) yield(); // wait
 }
 
+int serial3_room(void)
+{
+	uint32_t head, tail;
+
+	head = tx_buffer_head;
+	tail = tx_buffer_tail;
+	if (head >= tail) return TX_BUFFER_SIZE - head + tail;
+	return tail - head;
+}
+
 int serial3_available(void)
 {
 	uint32_t head, tail;