[RFR] Sensors should return some float values

Fixes https://github.com/rwaldron/johnny-five/issues/492.

Now sensors return also float values.

Question for the reviewer: what is the purpose of `analog` property in [sensor.js](https://github.com/rwaldron/johnny-five/blob/master/lib/sensor.js#L186)? Shouldn't the scale be also applied?