convert between RDF and iCalendar

 Resource Description Framework (RDF) is a standard model for data
 interchange on the Web.
 .
 RDF::iCalendar::Exporter takes some RDF using the W3C's iCalendar
 vocabulary, and outputs RDF::iCalendar::Entity objects.
 .
 An RDF::iCalendar::Entity objects is an individual iCalendar calendar.
 It overloads stringification, so just treat it like a string.