Usage of URN instead of URL for identifiers

[mkollenstart]

(Issue related to #163)

This issue is mainly for a discussion on how we should use the Information Model, as the usage of URN is already supported because it is a subset of URIs.

For the identifiers of especially ids:Participants and ids:Agents an URL identifier is not always the most concise way of identification. As the URL might not be actually reachable.

Is the usage of URNs a correct and recommended way of referencing using for instance an IDS namespace like urn:ids:participant:ID or urn:ids:agent:ID.