Tamogen Mosaic action which takes a folder of images as its input

Issues are:
- How big is the final image (mostly it has to be much bigger)
- How big are the individual photos (mostly smaller) and how they look like (white board, shadow, ...)
- Maybe nice to add some randomness to the grid: rotation and shift from origin
- Compare with existing collage software (eg picasa, ...)
- Method: reduce every small image to an average color value (fast) or use diff function to also take shapes in account (slow).

The choice of the right photo should be done after the random transformations.

Todo: this still requires a FolderField, use a CharField for now.