Conf queue

add CELERY_HAYSTACK_QUEUE conf to allow specification of which celery queue to use.

i run foreground and background queues. the foreground is for time-critical stuff and the background is for stuff that can stack up and take its time to complete. this will leave things going to default celery queue if you do nothing, but allows setting CELERY_HAYSTACK_QUEUE = '<queue-name>' to direct the celery haystack tasks to use an alternate queue (background in my case.)