Add `rels` to RESTful API entity-resources

Resources returning MOM instances now support access to the links the
instance participates in.

* The instance-resource provides a child-resource for each role, in
  which the instance can appear.

* The child resources are listed in the ``Link`` header.

* For requests wuthout the query argument ``RELS``, the response
  contains a list ``rels`` with the URLs of the child-resources.

* For requests with a ``RELS`` query argument, the response contains the
  links of the instance for all specified role (by default, for all
  roles on non-partial associations).