Support ordering of facets

[gsingers]

We should be able to control the order the facets are displayed by first inspecting from the pipeline the desired order and then following back to an order specified in the FUSION_CONFIG.js

[erikhatcher]

The easiest way (currently) would be to have a Set Params set facet_order=field2,field1,field2... kinda thing, and the UI picks up on that parameter in the Solr request params (in the response header).

However, I'm going to look into integrating my rules work into searchhub, and when that is in, we'll set a fusion/facet_order key in the response (outside the standard Solr response).

Either way, the list will come back in the Fusion response (optionally).