Support faceting in entities and relations listings. 

@scott2b I was wondering whether you might have a second to review this; it turned out to be more of a mess than I expected. 

Potential extensions: 

- [ ] Status facets
- [ ] 'relations' facet that combines inbound and outbound.
I'm going to be paranoid here and say maybe we should check to be sure there is a dot in the facet. If we want this to raise an exception, maybe there should be an InvalidPropertyFacet error instead of the generic IndexError