code: public TimezoneUrl setTimezoneUrl(String url) {
  		TimezoneUrl prop = (url == null) ? null : new TimezoneUrl(url);
  		setTimezoneUrl(prop);
  		return prop;
  	}
docs: Sets the timezone URL, which points to an iCalendar object that contains
  further information on the timezone.
  @param url the timezone URL (e.g.
  "http://example.com/America-New_York.ics") or null to remove
  @return the property that was created
  @see <a href="http://tools.ietf.org/html/rfc5545#page-106">RFC 5545
  p.106</a>
  @see <a href="http://tools.ietf.org/html/rfc2445#page-101">RFC 2445
  p.101</a>