Rework config and engines

I know, there's a lot of things changed in the Config role, but as we've talked with Sawyer, I think they make sense because they simplify a bit the way we manage the configuration.

There's a few bug fixes, some documentation clean-up, but this does not introduce any incompatibilities (as far as I know).

Dancer supports 4 kinds of engines: session, template, logger and serializers. They have a new attribute to store their own triggers, and there's another attribute for general config triggers.

(cc @xsawyerx)