Adds preferredAspectRatio instances object support

[mattaho]

In order to properly support including a preferredAspectRatio object in the image prop, I did a bit of a refactor of this component. Primarily this was to separate the concerns a little bit more, using a more functional style with return values and getting rid of the imageProps var, as well as separating things out into a few more functions.

Some of the tests had to be revised a little bit because they weren't quite testing for the correct behavior, and I added another test for the aforementioned preferredAspectRatio object. As a note, in the data, preferredAspectRatio exists as a sibling to aspect_ratios, not as a child. This is what necessitated the re-work

[kimthompson]

Honestly, I don't have any questions. This is really neatly done!