Revised message severities

 * checks that are arguable or retailer-specific are now SUPRESSED
 * checks that can be informative but do not represent a "should" rule
   in the spec are now USAGE, and the related message string reworded
   accordingly
 * in some rare cases, the severity level has been increased to match
   a "should" or "must" in the spec which was previously unchecked
   (e.g. `ACC_009`, `ACC_011`, `CSS_015`).
Fix tests for new message severities
Merge pull request #445 from IDPF/feature/revised-severities

Revised message severities