Added defines to ArduinoHardware.h to support Teensy 3.0 and 3.1

Added an extra #if to check for Teensy 3.x compiler flags (__MK20DX128/256__) when using the Teensyduino package and rosserial_arduino so that the USB connection is used.

Tested and works with Arduino 1.0.5 and Teensyduino 1.15-1.17 on Ubuntu 13.10 with ROS Hydro.