IMU and Temperature support

This is really just meant to be a spike to try some things out for the IMU/Accelerometer with separated controllers. I haven't extended it to Temperature or Gyro because I wanted to get some feedback first.

The Accelerometer has two kinds of controllers, a pin controller ("ANALOG") and a breakout driver for the "MPU-6050".  These breakouts live in `board.breakouts`.

I tested this with an analog Accelerometer as well as with the MPU-6050.
This reminds me: I need to add continuous read to Firmata.js