Add support for displaying a background image for a story

This adds support for associating a background image to a story, and displaying it in the story list and detail views. Right now, a random color or background image from a predefined list will be added to the story when its created.

This also adds a nifty parallax effect to the background image in the story detail view. It doesn't seem like the simulator supports the GPU, so rendering the parallax effect in the simulator can lag.