Merge remote-tracking branch 'cwebber/254_delete_queue_directories'

Merge remote-tracking branch 'cwebber/254_delete_queue_directories'

* cwebber/254_delete_queue_directories:
  Removing docstring bit about delete_file possibly deleting directories in the future
  Convert media processing backends to delete the queue directory (#254)
  Implement delete_dir in the FileStorage

Conflicts:
	mediagoblin/media_types/image/processing.py
	mediagoblin/media_types/video/processing.py

Conflicts, because those media_types already use the newer
proc_state.delete_queue_file() method (which needs
updating.