refactor dispatch

[chadwhitacre]

Refactor the dispatcher so that it doesn't depend on Aspen's request, response and website objects. I want to be able to use the dispatcher inside of Django.

Builds on #385/#386.

[chadwhitacre]

@pjz I have more to do here to fully modularize the dispatcher (removing references to Aspen request, response, and website objects), but the commits so far should leave the code in a stable state if you want to merge sooner rather than later.

[chadwhitacre]

Blech. Broke the tests. :-/

Outta steam for now, will pick up with this again later ...

[pjz]

I like the way this is trending. I've pondered trying to make the dispatcher usable by Flask before (since that's a very pure-WSGI stack); that might also be a worthy goal.

[chadwhitacre]

Okay! Ready for review, @pjz. :-)

I decided not to take out the dependency on Response. We're not trying to factor dispatcher entirely out of Aspen. A Django or Flask adapter will need to translate raised exceptions for each framework, and catching Response is as good as anything else. Raising fine-grained exceptions is #304; I think that's out of scope here.

[chadwhitacre]

Closing in favor of #388.