creating a write queue to ensure that only one operation is ever writing at once.
Class method and dispatch_once for write_queue
Reverting to completion on the main queue because [NSOperationQueue currentQueue] can return nil if not in an explicitly defined operation queue.
Merge pull request #30 from vokalinteractive/skillz_writequeue

Write Queue
Merge branch 'master' of github.com:vokalinteractive/CoreDataManager-iOS