Address developer experience around Job#job_type

[robacarp]

The current implementation for how to specify which queues a job should land in is opaque:

class GeneratorJob < QueuedJob
   def self.job_type
     "generator"
  end

  # ...etc
end

Should Job#job_type be renamed? Should a macro be provided to make that feature less verbose? At a minimum documentation should be added to the method so it's clear what it's supposed to do.

see #80 (comment)