Adds methods for tagging/untagging a properties datastream as canonical and a boolean method for checking if an object is the canonical representation.
Adds canonization methods to the common assignment helper.
Creation of master object now tags the created object as the canonical representation.
Characterization job now uses canonical? to determine if a content object merits thumbnailification.  Updated content_type factories to tag relevant factories as canon.
Merge pull request #77 from nu-lts/feature/canonical

Feature/canonical