forked from https://github.com/adjust/rmq

added:

-tasks can be assigned a priority number (to be processed sooner from "ready" queue)
-queues now hold ids of tasks (integers) instead of payloads
-introduced redis lua scripts for isolation
-option to run queue cleaner automatically in background once per minute
 will run once even with multiple number of connections (using distributed lock)
-added tests for priority, adjusted old tests to new functionality
-added example for priority, adjusted old examples to new functionality

todo:

-list items of each queue
-forwarding of delivery to another queue (alternative to push queues)
-use HMSET to store value & metadata
-calculate the stats inside single redis script to get precise counts
-delayed tasks
-recovery in case redis script fails in the middle (rare case)