Merge of high priority issue fixes and pull requests on top of version 0.6.2

I did merge in some important fixes and improvements from the community and our own deployment experience of Kue. This merge contains (merging, testing and fixing community code in pull requests):

DONE:
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FIX: Suppress "undefined" messages on String errors (https://github.com/LearnBoost/kue/pull/230)
FIX: Disable Search Indexes (redis memory leakage regardless of removing jobs) (related old issue is https://github.com/LearnBoost/kue/issues/58). I tried to merge with https://github.com/LearnBoost/kue/pull/218 but couldn't get it to work.
FIX: Cannot read property id of undefined errors (https://github.com/LearnBoost/kue/issues/252,https://github.com/LearnBoost/kue/pull/239)
FIX: Parameterize number of jobs limit in promotion cycles (https://github.com/LearnBoost/kue/issues/244)
FEATURE: Global graceful shutdown - handy on exiting on critical errors (https://github.com/LOKE/kue)
FEATURE: Pause/resume-ability of specific worker types - called within worker processors (https://github.com/LearnBoost/kue/pull/163)
Enhancement: Singleton Queue
FIX: Ensure event subscription before job save (https://github.com/LearnBoost/kue/pull/179)
FIX: failed event is called in first attempt (https://github.com/LearnBoost/kue/issues/142)



And there are a list of other important issues that can be merged in easily:
TO-BE-DONE:
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and these are seemed to be important and potential for next merges (ordered by simplicity)

FEATURE: Passing back results from job processor by job.data (https://github.com/SpiderStrategies/kue/commit/7defce249f59402706dd3280b0aa99083bce550d) (not sure kue really needs this... should be discussed)
Enhancement: Improving QOS, better timing, callback base job functions, ... (inspired by @dfoody extensions)
FIX: Issues when removing failed jobs (https://github.com/LearnBoost/kue/pull/99 and https://github.com/LearnBoost/kue/issues/197)
FEATURE: Delaying failed jobs (https://github.com/LearnBoost/kue/pull/227)