Support custom key prefix for jobs

Created by: kulbirsaini

Useful in larger deployments.

PR #177

Example:

var Queue = require('bull');

var videoQueue = Queue('transcode', 6379, '127.0.0.1', { keyPrefix: 'us_jobs' });
// Results in redis keys like
// us_jobs:transcode:id
// us_jobs:transcode:1231
var videoQueue = Queue('transcode', 6379, '127.0.0.1', { keyPrefix: 'in_jobs' });
// Results in redis keys like
// in_jobs:transcode:id
// in_jobs:transcode:1231