DATASOLR-564 - Add support for JSON facets.

[jlinn]

This change adds basic JSON faceting support. As noted in the commentary in NestedJsonFacet, there is a bug in Solr / SolrJ which prevents us from easily parsing the results of percentile facets. There is a proposed patch which should fix the issue, but it has not been merged yet. We could work around the issue to parse those results now, but it'd be pretty nasty.

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