Added optional generate param to generate_all_aliases

Hey,
I found it useful to be able to pass in the generate=True parameter to generate_all_aliases, e.g in some cases where I use this with a signal handler / async. task. Pretty straight forward and seems useful.

On a side note, I couldn't figure out a way to have automatic re-generation of thumbnails when I change the original image (e.g when original image ImageField is already populated and had thumbnails generated, if i change it by uploading new file, I cannot seem to get easy-thumbnails to re-generate thumbnails automatically based on the fact file is modified, when trying to use the saved_file receiver/signals as suggested in documentation).