Add UIColor+PerceivedLuminance-v1

The point of my last two pods because now you can do:

```objc
- (UIStatusBarStyle)preferredStatusBarStyle {
    return self.topImageView.image.averageColor.perceivedLuminance > 0.5
        ? UIStatusBarStyleLightContent
        : UIStatusBarStyleDefault;
}
```

Though I have discovered it is better to get the statusBarFrame amount of your image, so I'm deciding how I should go about submitting that pod.