code: public Categories addCategories(String... categories) {
  		Categories prop = new Categories(categories);
  		addProperty(prop);
  		return prop;
  	}
docs: Adds a list of keywords that describe the calendar.
  @param categories the categories to add
  @return the property object that was created
  @see <a
  href="http://tools.ietf.org/html/draft-ietf-calext-extensions-01#page-7">draft-ietf-calext-extensions-01
  p.7</a>