Add mark-down-ready event

Hey @robdodson,

This adds a new event, `mark-down-ready`, which is fired after the HTML representing the Markdown content has been added to the `#markdown` container. Pages that embed `<mark-down>` might find this event useful if they want to, e.g., [modify the `#markdown` children](https://github.com/jeffposnick/pageable-text/blob/master/pageable-text-source-markdown.html#L43) once they're available.

It also updates the Polymer dependency in `bower.json` to "^0.4.2".