Overhaul the documentation (#73)

* Overhaul the documentation

* Create a separate design doc, tf_job_design_doc.md that more fully
  explains the design and motivation.
* Create a separate developer guide with information about building the
  repository and other information specific to developers/contributors.

* Rewrite README.md to focus on information relevant to users of TfJob.
  Add a lot more information about how to monitor jobs as well as information
  explaining how it works.

* * Instructions should install package from URL.