Enable custom error handlers to be specified and used

This PR adds support for custom error_handlers to be specified. It also adds some documentation on how to use this feature on a global application level, so that all error pages are consistent across the application.

Obviously, you can always implement custom error pages on a per-handler basis. The motivation behind this PR, as specified in issue #124, is to allow the basic 404 page to be as good looking as the rest of your application's error pages.

I successfully tested the changes against my only Bottle-style app, and it seems to work properly. I can't vouch for regular Cyclone-style applications, as I don't have any of those handy, but by using the mixin properly, there is no reason it wouldn't work.