The constants in this interface define the different types of resource references that
are declared in RFC 3986: http://tools.ietf.org/html/rfc3986
We are using the term "URL" instead of "URI" as this is more common in web applications
and we do not need to distinguish them as the difference is mostly semantical and
less technical. Generating URIs, i.e. representation-independent resource identifiers,
is also possible.