Add optional close button; minor cleanups

[driehuis]

Add a close button, inspired by the patch in #20, but with an example close button added to the arrows sprite, and (commented out) code in index.html that shows how to activate the close button. By default, the close button is not shown.

Also:

• clean up whitespace (two lines were misindented, probably as a result of an earlier pull request).
• Return a value consistently from showImage, hideOverlay and showOverlay, to quell error messages in the Javascript console on Firefox.
  Feel free to cherrypick from the commits (I wasn't aware that github pools new commits into an existing pull request).