Add schema for international development funds

Adds a schema for international development fund documents to be published by DFID.

The schema will be added to Rummager as well.
I assume this will become a field named "sector" in rummager.  I'm not wild about that, since I'd quite like to rename the "speciailst_sector" core field to "sector" in future (probably; ie, unless we rethink all our terminology in the near future).  Conflicts between different document types don't matter much, but conflicts with core field names do.

How about "idf_sector" or something similar.