bake: Make pipeline processing multi-process. Not many changes here, as it's pretty straightforward, but an API change for processors so they know if they're being initialized/disposed from the main process or from one of the workers. This makes it possible to do global stuff that has side-effects (e.g. create a directory) vs. doing in-memory stuff.

cov-core==1.15.0
coverage==3.7.1
mock==1.0.1
pytest==2.7.0
pytest-cov==1.8.1
pytest-mock==0.4.3