Remove need for mixin, default all jobs to single queue by default

Fixes two issues:

- [x] Support jobs without need for queue mixin #26 
  - Now a job is any object with a perform method
  - Queue mixin is now totally optional syntactic sugar
- [x] By default, have all jobs enqueued to the same app-specific tube #27 (my.app.namespace.backburner-jobs)
  - This is configurable by overwriting the `primary_queue` setting
  - All jobs go here unless otherwise specified

@bradgessler can you verify this is what you had in mind?