fixed infinite recursion error for long lists

 * epub3-to-dtbook.xsl
   * fixed too deep recursion when calculating value for li elements ("infinite" recursion error)
   * fixed 'class attribute on a noteref was dropped' warning
   * skip warnings when dropping http-equiv, charset and viewport
 * dtbook-to-epub3.xsl
   * removed class 'list-preformatted'
Merge pull request #126 from nlbdev/infinite-recursion-error-li-value

epub3-to-dtbook: fixed infinite recursion error for long lists