Webservice: getRelatedAssociations() (#707).

The `Topic`'s `getRelatedAssociations()` method is RESTful:
{{{
GET /core/topic/{id}/related_assocs
}}}
Result is an array of associations (may be empty).

To filter the result set the usual 4 traversal query parameters can be applied (all optional):
{{{
assoc_type_uri
my_role_type_uri
others_role_type_uri
others_topic_type_uri
}}}

Note: the `include_childs` query parameter is not yet supported.

See #707.