Introducing resque-unique_at_runtime

Ensures that only one job for a given queue will be running concurrently. A semanticaly versioned Resque plugin which ensures for a given queue, that only one worker is working on a job at any given time. Resque::Plugins::UniqueAtRuntime differs from resque-lonely_job in that it is compatible with, and can be used at the same time as, resque-solo. Resque::Plugins::UniqueAtRuntime differs from resque_solo in that resque-solo offers queue-time uniqueness, while resque-unique_at_runtime offers runtime uniqueness.