Code review and patches of the deferred functionnality
 * There was no tests of the "deferred" functionality, no there are a few
 * The deferred structure was including the queue with the job_id needlessly and that induced a bug when releasing a job into another queue than the original one
 * Addition of an helper method on the queue object to release a deferred job into that queue.
 * The "blocked_by" keyword seems useless and might result from a misunderstanding of the "depends_on" keyword, it will be removed after further review to comfirm that fact.
Merge branch 'wip-queue' of github.com:glenfant/rq into wip-queue