Add change tracking fields to Artefact

This adds two new fields to the Artefact model, to contain the 
latest change note for a content item, and the public timestamp at
which it was last changed.

These will be included in the document that is sent to Rummager.
Merge pull request #251 from alphagov/add-public-timestamp-and-latest-change-note-to-artefact

Add change tracking fields to Artefact