Adds isOn property to RGB

Adds isOn property for RGB that looks to see if ANY of the colors are
on. When calling .on(), it leaves the led as-is if it’s already on.

Also adds chaining capability to .on() and .off() and saves color state
when those functions are called.

Does not yet address saving previous color as requested in #375

Other functions like toggle/brightness/etc still need to be updated to
treat RGB as a whole rather than individual LEDs.
I think you can eliminate the forEach and `isOn` var: 

```js
return Led.RGB.colors.some(function(color) {
  return !!state[color];
});
```