Prepare for 1.0.0

  * celery async support, as a it didn't fulfill the dream in that incarnation.
  * removed fail_on_error, as the use case is muddled.
  * remove the multiple bus support offered by app.py; making it the lib user's responsibility
  * removed the request/session confundary, and now there is only session, where session is defined as everything that happens in a single top level invocation. The session is a bucket of values. 
  * Tighten all the error handling.
  * Organize bus.py