core: Start of the big refactor for PieCrust 3.0. * Everything is a `ContentSource`, including assets directories. * Most content sources are subclasses of the base file-system source. * A source is processed by a "pipeline", and there are 2 built-in pipelines, one for assets and one for pages. The asset pipeline is vaguely functional, but the page pipeline is completely broken right now. * Rewrite the baking process as just running appropriate pipelines on each content item. This should allow for better parallelization.

--- a/piecrust/templating/jinjaengine.py	Sat Apr 29 21:42:22 2017 -0700
+++ b/piecrust/templating/jinjaengine.py	Wed May 17 00:11:48 2017 -0700
@@ -1,6 +1,6 @@
 import os.path
 import logging
-from piecrust.environment import AbortedSourceUseError
+from piecrust.sources.base import AbortedSourceUseError
 from piecrust.templating.base import (TemplateEngine, TemplateNotFoundError,
                                       TemplatingError)