Brightness Methods: getBrightness, isLight, isDark

TinyColor is using the W3C's perceived brightness formula when calculating readability:

```
tinycolor.readability = function(color1, color2) {
    var a = tinycolor(color1).toRgb();
    var b = tinycolor(color2).toRgb();
    var brightnessA = (a.r * 299 + a.g * 587 + a.b * 114) / 1000;
    var brightnessB = (b.r * 299 + b.g * 587 + b.b * 114) / 1000;
    var colorDiff = (
        Math.max(a.r, b.r) - Math.min(a.r, b.r) +
        Math.max(a.g, b.g) - Math.min(a.g, b.g) +
        Math.max(a.b, b.b) - Math.min(a.b, b.b)
    );

    return {
        brightness: Math.abs(brightnessA - brightnessB),
        color: colorDiff
    };
};
```

This PR adds a `getBrightness()` method using the above brightness formula.  This is useful as you can now get the perceived brightness of any TinyColor instance.

I also added 2 helpers `isLight()` and `isDark()` allowing:

```
var background = tinycolor('#333');

if (background.isLight()) {
    ...use a dark overlaying color...
}
```