Facets API: add getFacettedTopic() method (#652).

The Facets API has a new RESTful method:
{{{
Topic getFacettedTopic(long topicId, List<String> facetTypeUris)

GET /facet/topic/{topic_id}?facet_type_uri=<uri1>&facet_type_uri=<uri2>&...
}}}

If returns a topics (including its composite value) along with its facets values according to a list of Facet Types.

See #652.