Added zenqueue backend as zenqueued based heavily on a patch by Daniel Lindsley. To use the zenqueued backend you'll need to set QUEUE_BACKEND to zenqueued, and also set QUEUE_ZENQUEUE_CONNECTION. The zenqueued backend defaults to using the client for HTTP but you can use either the HTTP or native method by setting QUEUE_ZENQUEUE_METHOD to http or native. Note that zenqueue does not allow for multiple named queues, so the name argument to the Queue constructor is ignored. Several other options (__len__, delete_queue, and get_list) are not supported by zenqueue.