Add change notes to manuals

This will power pages equivalent to:
http://www.hmrc.gov.uk/manuals/admlmanual/updates/updateindex.htm

The format is designed to be as close as possible to the Specialist Publisher
change notes format, with the addition of section_id.
Merge pull request #57 from alphagov/add_change_notes_to_manuals

Add change notes to manuals