update for issue #22 and change 'attribute' to 'property' in text.

 * handles issue #22 (default could be either the JSON or XML variant)
 * changes all refs to 'attribute' (very XML-ish) to 'property' (more JSON-esque).
 * fixes a bug in the full-examples that placed the image URL in the `value` field by mistake. this introduces the example of both `url` and `value` properties on the same `data` element.