Extends #1148: article modification date from FS.

  - This pull request adds a new attribute to the article object: mdate.

  - article.mdate retrives the article file last modification date from the
    fylesystem (os.stat(full_path)), thus providing article modification date
    automatically.

  - This extends #1148 in the sense that an author doesn't have to add the
    :modified: metadata to each article, as she can just edit the template
    files to display the new article attribute.

  - Documentation updated to describe the new feature.