Class based views

[tomchristie]

Taken from #35, with a little bit of extra work on top. Huge thanks to @ERM for breaking the back of this one!

Views are now instantiated per-request. Use them either as regular ASGI apps, or route to them with @app.route(...).

We'll also want to add an equivalent for websockets that looks like...

class App(...):
    async def on_connect(self, websocket, **kwargs):
        ...
    async def on_receive(self, websocket, data):
        ...
    async def on_disconnect(self, websocket):
        ...

I'm not totally sure about the naming. We could go for HTTPEndpoint/WebSocketEndpoint instead of "views".

[jeffbuttars]

I like the idea of using '*Endpoint' over 'views'. That's how I talk about API implementations in IRL.

[tomchristie]

@jeffbuttars Indeed. I've included it in the list of potential renames in #51