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`.

*.pyc
*.egg-info
.cache
__pycache__
venv
tags
bower_components
node_modules
build/lib
util/messages/_cache
util/messages/_counter
dist
docs/_cache
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foodtruck/static
piecrust.egg-info
piecrust/__version__.py
.ropeproject