Move the rendition vocab from default to reserved

 * the 'rendition' prefix is still reserved and predefined
 * the 'rendition' prefix is allowed to be redeclared explicitly
   (with a WARNING)
 * the Renditio vocab URI may be associated to another prefix (with a
   WARNING)

Take the opportunity to better separate the vocabs computed for each
"class" of properties (`meta` & `scheme`, `item`, `itemref`, `link`).
Merge pull request #451 from IDPF/feature/issue-437

Move the rendition vocab from default to reserved
Improve EDUPUB heading SCH rules
Simplify XPath expressions in edu-headings.sch
Merge pull request #440 from IDPF/feature/edupub-headings

Improve EDUPUB headings rules
Do not parse collections 'link' as new OPF items

A new `OPFItem` was created for each `link` element. This PR makes
sure this does not happen with the new `link` element introduced as an
allowed child of the new EPUB 3.0.1 `collection` element.
Merge pull request #452 from daisy/feature/issue-443

Closes #452
Remove check OPF_023

`OPF_23` was checking that there was only one `metadata` element in
the OPF, but:

  * this is already checked by RelaxNG schemas
  * the check is flawed since `collection`s also allow `metadata`
    children
Merge pull request #450 from idpf/feature/remove-OPF_023

Closes #450
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
Check XHTML Content Doc extension in EPUB 3

  * make sure extension is '.xhtml', in `OPFChecker30`
  * remove duplicate `checkItem()` call in `OPFChecker`
  * Only report `HTM_014` for EPUB 2 (in `EpubHTML5StructureCheck`)
  * Add a new test for `HTM_014a`
  * Add a new test for `HTM_014`
  * Fix existing tests

Fixes #447