bake: Several bug taxonomy-related fixes for incorrect incremental bakes. * Improve how the baker processes taxonomy terms and figures out what needs to be re-baked or not. * Create bake entries for clean taxnomy terms so they're not deleted by an incremental bake. * Add more information to bake records. * Slugify taxonomy terms is now done by the route in one place. * Fix a bug where the cache key for invalidating rendered segments was not computed the same way as when the caching was done. * Fix how term combinations are passed around, rendered, printed, parsed, etc. (TODO: more word needed in the routing functions) * Expose to the template whether a taxonomy term is a combination or not. * Display term combinations better in the built-in theme. * Rename `route.taxonomy` to `route.taxonomy_name` to prevent confusion. * Add options to show bake records for previous bakes.

class Event(object):
    def __init__(self):
        self._handlers = []

    def __iadd__(self, handler):
        self._handlers.append(handler)
        return self

    def __isub__(self, handler):
        self._handlers.remove(handler)
        return self

    def fire(self, *args, **kwargs):
        # Make a copy of the handlers list in case some handler removes
        # itself while executing.
        handlers = list(self._handlers)
        for handler in handlers:
            handler(*args, **kwargs)