Make create_content_thumbnails more generic, allow for $options['rotate']

Three changes combined here (sorry!):
1) (cosmetic) - remove "TODO" for Biopix exception
2) allow for options['rotate'] to be passed to grab_file, which overrides any EXIF rotation info. This should enable future changes to the flickr harvesting routine to do the right thing when it encounters images rotated in flickr.
3) make create_content_thumbnails loop over the various default_image_dimensions, rather than hard code each type.

If this is not accepted, note that some of the changes under point (3) need implementing anyway (e.g. the first pass of create_smaller_version() should work on the value returned from reduce_original(), not on the passed-in filename).