Class for creating delayed jobs that can be de-duped with existing delayed jobs already in the delayed jobs table. You just specify some additional columns on your delayed_jobs table and set them to have uniqueness constraints. Then specify these column values when you create a UniqueDelayedJob and if a duplicate key is raised on insert, then the insert will just be ignored. There are factory methods for creating a delayed job in the following ways: * with a delayed job handler class (one that responds to perform()) * with an object, method and method arguments * with a code string to be evaled NOTE: you must have delayed_job installed as a gem or plugin