- Drop python2.6 support.

- For the find-untranslated feature, add the possibility to mark a tag to be
  ignored by setting the "i18n:ignore" attribute on the tag. Also works for
  attributes with the "i18n:ignore-attributes" attribute.

- Reviewed by Maurits van Rees.

- When using Chameleon, you will need to expand the chameleon.i18n.WHITELIST
  in order to work with the newly introduced attributes.