Create DFID importer to import...DFID reports!

 * Create DFID import entrypoint, attachment mapper/mapper.
 * Add import_dfid_reports script and rename aaib reports script
 * Extract method refactor on AaibImport
 * Create DocumentImport::AttachmentHelpers to extract common utility
   methods for attachment mappers.
 * AAIB fixtures now live in `spec/fixtures/aaib_import`
Merge pull request #194 from alphagov/import-dfid-reports

Create DFID importer to import...DFID reports!
Fix pagination count

The number of records in the system is the number of document_ids, not
the number of editions.
Merge pull request #195 from alphagov/fix-pagination-count

Fix pagination count
Remove base path concept from attachment
Merge pull request #197 from alphagov/dfid-import-bug

Allow explicit not relative paths to import content