internal: Worker pool improvements - Slightly more correct multi-processing code. - Handle critical worker failures. - Correctly setup workers when run from `py.test`. - Add support for using `msgpack` or `marshall` instead of `pickle`.

--exclude=build
--exclude=dist
--exclude=docs/_cache
--exclude=docs/_counter
--exclude=bower_components
--exclude=node_modules
--exclude=docs/raw
--exclude=*.egg-info
--exclude=__pycache__
--exclude=__tmpfs__