(RSS::Element.install_have_child_element,
   RSS::Element.install_have_children_element,
   RSS::Element.install_text_element,
   RSS::Element.install_date_element): call
  RSS::Element.install_model internally.

* lib/rss/0.9.rb: followed new API.
* lib/rss/1.0.rb: ditto.
* lib/rss/2.0.rb: ditto.
* lib/rss/content.rb: ditto.
* lib/rss/dublincore.rb: ditto.
* lib/rss/image.rb: ditto.
* lib/rss/syndication.rb: ditto.
* lib/rss/taxonomy.rb: ditto.
* lib/rss/trackback.rb: ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@10341 b2dd03c8-39d4-4d8f-98ff-823fe69b080e