Async

Processing the image take a few seconds depending on the image size.

I've setup a async method for resizing the image and processing, makes it easier to use
it from the main thread.

I've also added a "random color threshold", which you had hardcoded to 2. There were a few album covers that, when resized, would not be able to detect a background color and would crash. I've also added a check for when the background color is nil.