core: Continue PieCrust3 refactor, simplify pages. The asset pipeline is still the only function pipeline at this point. * No more `QualifiedPage`, and several other pieces of code deleted. * Data providers are simpler and more focused. For instance, the page iterator doesn't try to support other types of items. * Route parameters are proper known source metadata to remove the confusion between the two. * Make the baker and pipeline more correctly manage records and record histories. * Add support for record collapsing and deleting stale outputs in the asset pipeline.

--- a/piecrust/events.py	Wed May 17 00:11:48 2017 -0700
+++ b/piecrust/events.py	Sun May 21 00:06:59 2017 -0700
@@ -1,5 +1,7 @@
 
 class Event(object):
+    """ A simple implementation of a subscribable event.
+    """
     def __init__(self):
         self._handlers = []