CHANGED methods now returning thunks instead of promises

Methods like `once`, `reset` and `push` now return thunks instead of
promises.
Disappointing for some, but easier for those of us using
[Mocha](http://visionmedia.github.io/mocha/) and
[co](https://github.com/visionmedia/co).
Cleaned up run_job
Session setup now uses generators

And in doing so detect issue where default tube was not ignored
Cleaned up error handling and workers start/stop
Proper test case for continous processing
release 0.7.3