I2C Motor Control support

**API for MotorInterface class**

```
__init()__
get_throttle() -> float
get_throttle_raw() -> int
get_regen() -> float
get_regen_raw() -> int
```

All `get` functions return the most recent value. The `raw` functions return an int in [0, 256], and the non raw functions return a double in [0, 1]. All should return `None` if the values haven't been set yet. 