Refine save button behavior

Toggle the ``disabled`` attribute on the button when it is displaying the
"Saved" message.

Use a modified version of the
[JavaScript Pretty Date] (http://ejohn.org/blog/javascript-pretty-date/)
function to format the timestamp in human readable form.

Use ``setInterval`` to continually update the timestamp.

Remove methods and code which wrote the timestamp to a separate element.

Addresses #548