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.

--- a/piecrust/processing/compass.py	Sat Jun 20 19:16:38 2015 -0700
+++ b/piecrust/processing/compass.py	Sat Jun 20 19:20:30 2015 -0700
@@ -1,7 +1,6 @@
 import os
 import os.path
 import logging
-import platform
 import subprocess
 from piecrust.processing.base import Processor, PRIORITY_FIRST
 from piecrust.uriutil import multi_replace