Adds XHTML Ruby support. Ruby texts will be rendered above the base texts,
as a small text. This supports both simple ruby and complex ruby of W3C spec.

Testcases:
* http://www.w3.org/International/tests/sec-ruby-markup-1
* http://www.aozora.gr.jp/cards/000035/files/1567_14913.html
* http://www.keishicho.metro.tokyo.jp/sikumi/pipo/music/music.htm

WWW: http://piro.sakura.ne.jp/xul/_rubysupport.html.en