Importer set up initially to import AAIB reports

From `import/metadata` (json files, see spec/fixtures/import) and
`import/downloads`, import AAIB reports.

Also create an optional decorator used to validate that all attachments
for a document have been referenced in the body of that document.

Worked on by myself and @bestie
Handle duplicate titles.

 * For documents which already have a newer version, just skip it.
 * For documents which have an older version, delete that version and
   import the newer instead.
Strip PDF tags from AAIB body on import
Fix #update bug in DocumentMetadataDecorator

The extra attributes hash was completely replaced
on update with whatever was in the given hash.

This required the caller to provide all the
key/value pairs for the extra attributes or have
them wiped out. This was inconsistent with the
update behaviour of the other fields.
Import all the AAIBS