Altered Delayed::Command to work with or without Rails

When running the delayed_job daemon in a codebase without Rails, we've had to create a fake Rails module to provide the logger and root methods that Delayed::Command expects. Given the rest of the delayed job code can run without Rails, it would be nice if the Command behaved the same way.

Please let me know if the tests are sufficient or not. Given the entire class was lacking a test, I have only added tests for the code that I altered.