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.
Need to support `opts.device` being an unknown device object: 

```js
var device = null;
if (opts.device && typeof opts.device === "string") {
  device = Devices[opts.device.toUpperCase()];
} else {
  device = opts.device;
}

if (device === null) {
  device = Devices["ANALOG"];
}
```


Note: this is probably inconsistent elsewhere, but those are bugs to be fixed (I just spotted this in `Compass`, which I'm fixing presently)