Update asciidoc_setXhtmlRendition function from render module.
    
    - Previously, xhtml files were produced as result of xsltproc
      transformation and no markup cleaning was performed after it. As
      result, the final html output appeared unindented.  This update
      changes the asciidoc_setXhtmlRendition function to use Dave
      Raggett's tidy program to indent the html output and perform markup
      cleaning.