Webservice: getAssocRelatedAssociations() (#707).

The `Association`'s `getRelatedAssociations()` method is RESTful:
{{{
GET /core/association/{id}/related_assocs
}}}
Result is a 2-property object:
{{{
{
    "items": [      # array of associations (may be empty).
        ...
    ],
    "total_count": ...
}
}}}

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

The `include_childs` query parameter is supported as well.

The Core's traversal API including its REST API is now fully orthogonal.

See #707.