Add international_development_fund schema

This format will be published by DFID editors via Specialist Publisher.

See also: https://github.com/alphagov/finder-api/pull/33
These £-signs are causing preview search to be broken.  They're not US-ASCII, of course, but rummager is trying to parse them as such.  Errbit reports are here: https://errbit.preview.alphagov.co.uk/apps/539835ab0da1151215000888