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

- [x] Add tests to cover each of the scenarios in the linked ticket (only `from` or `to` provided, rejection of multiple parameters)
Your comment pretty much sums this up perfectly. Adding a method onto the filter object feels like the more natural solution, but I think we're better off sticking with this smell in order to keep all the elasticsearch query generation in this class.