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