Add Keyboard.releaseAll() for Teensy3

teensy3/usb_keyboard.c

 
 	anybits = keyboard_modifier_keys;
 	keyboard_modifier_keys = 0;
+	anybits |= keyboard_media_keys;
+	keyboard_media_keys = 0;
 	for (i=0; i < 6; i++) {
 		anybits |= keyboard_keys[i];
 		keyboard_keys[i] = 0;

teensy3/usb_keyboard.h

 	void send_now(void) { usb_keyboard_send(); }
 	void press(uint16_t n) { usb_keyboard_press_keycode(n); }
 	void release(uint16_t n) { usb_keyboard_release_keycode(n); }
-	void releaseAll(void);
+	void releaseAll(void) { usb_keyboard_release_all(); }
 };
 
 extern usb_keyboard_class Keyboard;