serve: Refactoring and fixes to be able to serve taxonomy pages. * Page sources' `findPagePath` is renamed to `findPageFactory`, so that it also returns source metadata. * Page refs now store possible hits more properly, and use the previous point to also store metadata. As a result, they can also return a proper factory. * Implement `findPageFactory` correctly in all built-in sources. * When the Chef server matches a taxonomy page, get the source metadata from the page ref in order to make a more proper page. * Make the `getRoute(s)` functions explicitely say if they want taxonomy routes or not.

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)