title: Django-Filebrowser cannot create a version with auto width and constrained height?
body: <p>I've been using Django-Filebrowser and setting up my versions <a href="http://django-filebrowser.readthedocs.org/en/latest/settings.html" rel="nofollow">as specified in the docs</a>, which indicate you specify image versions like so:</p>
  
  <pre><code>VERSIONS = getattr(settings, "FILEBROWSER_VERSIONS", {
      'thumbnail': {'verbose_name': 'Thumbnail (1 col)', 'width': 60, 'height': 60, 'opts': 'crop'},
      'small': {'verbose_name': 'Small (2 col)', 'width': 140, 'height': '', 'opts': ''},
  })
  </code></pre>
  
  <p>...note the 'small' version size with an unspecified height, which successfully generates an image version with a constrained width and an auto height. This works successfully for me, however the reverse does not (constrained height but auto width):</p>
  
  <pre><code>'auto_width': {'verbose_name': 'Auto Width', 'width': '', 'height': 140, 'opts': ''},
  </code></pre>
  
  <p>Is there a trick here that I'm missing, or does the library not have this ability?</p>