HOLD: Publish date facet

A new facet class that handles filtering by post publish dates. This 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. Custom order the date ranges so they're sorted by dates rather than by count or name, which're the only choices currently.