Support data tag with the same name as command tag

[niuxuetao]

Introduce xml namespace "data" to identify a tag as data tag regardless of the tag name. For instance:

<data id="x">
  <!-- this will not open browser, it will define a property called "url" -->
  <url xmlns="data">http://google.com</url>
</data>

use xmlns="" to mean command tag again within a differently namesapced parent tag

[niuxuetao]

done in r1.8