Date filtering

Implements date range filtering for unified search. Done in the form of:

`filter_opening_date=from:2013-01-01,to:2013-03-31`

Ticket: https://trello.com/c/4iReFd0W/293-rummager-add-support-for-date-filtering-1

Also extends the behaviour of `ALLOWED_FILTER_FIELDS` to allow filtering by any field in the document type schema.

### TODO

- [ ] Add tests to cover each of the scenarios in the linked ticket (only `from` or `to` provided, rejection of multiple parameters)
Maybe it's just my python background, but I rather dislike this pattern of hiding the fact that schema_mappings is a class member.  I'd much rather access it explicitly as @schema_mappings, so I know what I'm dealing with.  What's the argument for using attr_reader here?