Tweak Celery Task

- Make sure Exceptions are pickleable (not sure if this was not the
  case but this is the pattern as documented in the celery docs.
- Don't create a task_id in the GMG code, but save the one
  implicitely created by celery.
- Don't create a task-id directory per upload. Just store queued uploads
  in a single directory (this is the most controversial change and might
  need discussion!!!)

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Switch deprecated BROKER_HOST to new BROKER_URL

People will need to switch that setting in their mediagoblin.ini too.
Make sure we RELEASE NOTE this.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Remove specific celery version requirement

Any celery above 2.5 should work just fine.
 -update to latest master
- have mg generate task_id

remove
- need self.metadata with BaseProcessingFail
- pass feed_url into ProcessMedia run()