Incorporate Wikimodel in XWiki sources

See http://markmail.org/thread/sdbom3d4rlwatx7s

Motivation:

• Wikimodel has been inactive for years (since 2009). Actually that's not quite
  true, there's been one developer working on it regularly, it's Thomas Mortagne…

• We heavily depend on wikimodel in the XWiki Rendering module (for our syntax
  parsers) which is a key module for XWiki

• It's more difficult for us to contribute to the wikimodel project since it
  means:
  • committing in a different project with different rules
  • there's nobody doing releases on the wikimodel projet and we need the
    releases to be synced with our releases since otherwise we cannot release on a
    SNAPSHOT dependency
  • there's no community there so it's not fun and doesn't help for quality
    control/reviews/etc
  • since we push XWiki Commons and XWiki Rendering to Maven Central we also need
    the wikimodel releases to be pushed to Central which is not happening now

• The wikimodel project has a different scope than our need. Mikhail (owner and
  admin of Wikiodel - not active since 2009 - some commits here and there) wanted
  it to remain only for wiki syntaxes. We added support for HTML parsing in it but
  Mikhail never liked it and wanted us to move it to XWiki.

• We have some impedance mistmatch between the wikimodel model and the xwiki
  rendering model which causes us to do some circumvolution in the code which
  leads to issues still beeing open in our JIRA (they've been opened for a long
  time now)

• We believe wikimodel would benefit from a larger and active community within
  the XWiki ecosystem. Wikimodel has been stagnating for years and we'd like it to
  live on and evolve.