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.
String body substitutions (PDF image) on AAIB reports
Import all the AAIBS
Update report type mappings and schema
Improve import logging

* Log the filename
* Log duration in ms
* Log errors when encountered (and continue)
* Allow arbitrary data to be logged
Correctly map aircraft categories to schema fields
AAIB import mapper strips leading whitespace
Update schema
Replace underscores in attachment titles for AAIB import

This is a work-around for a bug in Kramdown which causes the following

[a_1.pdf](http://example.com/a_1.pdf)

to produce:
1: <p><a rel="external" href="http://example.com/a_1.pdf">a_1.pdf</a></p>

but got:
1: <p>[a<em>1.pdf](http://example.com/a</em>1.pdf)</p>