code: public RelationshipType getRelationshipType() {
  		String value = first(RELTYPE);
  		return (value == null) ? null : RelationshipType.get(value);
  	}
docs: <p>
  Gets the RELTYPE (relationship type) parameter value.
  </p>
  <p>
  This parameter is used by the {@link RelatedTo} property. It defines the
  kind of relationship the property is describing (for example, a "child"
  relationship).
  </p>
  @return the relationship type or null if not set
  @see <a href="http://tools.ietf.org/html/rfc5545#page-25">RFC 5545
  p.25</a>