Removed the CallbackLayer layer.

Probably not a great idea to have this kind of functionality available considering Veto is supposed to be teaching good style. Endless chains of brittly-interacting callbacks are never nice.
Added the ability to match routes on HTTP method too.

* Implemented a separate Route class.
* Implemented a general interface for layers.
Add an exception class for missing configuration options.
Merge branch 'master' of git://github.com/themainframe/veto