Publish date facet

For https://github.com/GigaOM/legacy-pro/issues/4087

Implemented a new facet class that filters search results by post publish dates. As part of this feature, I added a facet widget sort order option of "None", which will prevent scriblio from sorting the facet list it receives from the facet class. In Facet_Publish_Date class' case, it will retain the date range ordering as declared in that class.

The feature is working but I still need to add a couple of things:
   1. handle a base URL of `/date-range/<some_date_range>/` when no other facet is selected. This current just displays S.go's home page but it should probably display one of the search results templates.
   1. we may need to add some additional rewrite rules -- currently this wijax url which's used by the Facets widget in the left sidebar of S.go does not work correctly (it returns an HTML page rather than a JSON payload): http://search.local.gostage.it/date-range/yesterday/wijax/c17954cffebe0093463edd966a1c7f95/?company=intel, but if the url is formed this way then it works: http://search.local.gostage.it/company/intel/wijax/c17954cffebe0093463edd966a1c7f95/?date-range=yesterday
`$tag_key => $tag_val` might be easier to understand here.