code: public Url setUrl(String url) {
  		Url prop = (url == null) ? null : new Url(url);
  		setUrl(prop);
  		return prop;
  	}
docs: Sets a URL to a resource that contains additional information about the
  journal entry.
  @param url the URL (e.g. "http://example.com/resource.ics") or null to
  remove
  @return the property that was created
  @see <a href="http://tools.ietf.org/html/rfc5545#page-116">RFC 5545
  p.116-7</a>
  @see <a href="http://tools.ietf.org/html/rfc2445#page-110">RFC 2445
  p.110-1</a>