Encapsulate image processing inside mcSkin struct

Adjusted some stuff. Image processing is done within a struct, mcSkin, which inherits from minecraft.Skin. This is slightly "nicer" design, I believe, and more importantly allows us to easily store information about the image we're processing.

I originally came here to make an isometric head view for this day of [24pullrequests](http://24pullrequests.com). This paves it way for that: it allows us to, for example, gracefully handle more fine-tuned resizing of avatars (soon to come!) beyond the currently always-on nearestNeightbor, without adding additional complexity inside the http mux.
I'm not going to pretend I fully understand the changes, so please forgive me if I ask a stupid question.

Is there a reason to not use the GetHead function here? I replicated the format you changed it to in the GetBody function:

	err := skin.GetHead()
	if err != nil {
		return err
	}
	headImg := skin.Processed

And that works correctly for me.