Allow picture render size to be flexible in width or height

Before, you had to state render sizes in the format '300x200'.

Now, you can use e.g. '300' for inferring the height from the image file preserving the aspect ratio (or 'x200' for inferring the width).

These two additional ways of specifying the image geometry are provided by the used image processing library Image Magick (and its wrapper dragonfly), see http://www.imagemagick.org/script/command-line-processing.php#geometry.

The only problems with stating render sizes in the form of '300' or '300x200' were the thumbnail and cropping tool box in the admin interface; the image processing worked correctly out of the box in the frontend.