Implement an I2C I/O expander to drive LEDs

I've made a hardware change in the robot to drive the Light Plant
LEDs via an I2C I/O expander, the PCF8575 on a breakout board from
Sparkfun.  This change will allow me to drive up to 16 LEDs without
major hardware changes.  Additionally, this opens up GPIO pins on
the Arduino for other functions.