Add start-server and stop-server commands.

* Add "oven", the simple biscuit http server (intended for development) which "bakes" a biscuit
  with each request.   If an error occurs during the baking process, that error can be sent to
  the browser, thus preserving the traditional change + "reload" pattern of development.
* Add mechanism by which various "ovens" can be started and stopped as independent child processes.
* Adjust cmd.js to handle promises returned by various commands and control output / exit status
  accordingly.