Add support for being an SPI slave, and transmitting Euler angles to an SPI master

 * Data is normally sent out from the Razor over the serial port, which is
 * not always convenient for embedded applications.  The Razor I2C hardware
 * is already in use, and software implmentations (on other pins) are not
 * fast enough.  However, most boards have an SPI interface which is often 
 * not used. E.g. the Leonardo does not share the SPI pins with other digital
 * pins.  This modification allows the Razor to be an SPI slave.