mod_base: Add 'md5' filter to translate a string to an md5 hex value

This filter is useful for generating md5 hex values for use with services such as Gravatar.

Example use case :

     {% with m.rsc[id] as user %}
         <img src="http://www.gravatar.com/avatar/{{ user.email|md5|lower }}" alt="{{ user.username }}"  />
     {% endwith %}