Added show_on_appear option, ideal for progressive jpeg format

Added the show_on_appear option to make images to be shown while they're loading. Ideal setting for progressive jpeg format. See attached image.

![ff62c00a-637f-11e4-9c97-d9daaca0a5bb](https://cloud.githubusercontent.com/assets/1127721/4892277/89a1ea90-63b0-11e4-8dd2-8940d9ee895d.png)