Add WSGI and ASGI dispatchers #49

We should allow clients to trivially plug into WSGI and ASGI apps.
This will allow:

• Using httpcore as a test client.
• Stubbing out third-party-services during tests, rather than making live network requests.

We should support the following:

def hello_world(environ, start_response):
    ...

client = Client(app=hello_world)

And...

async def hello_world(scope, receive, send):
    ...

client = Client(app=hello_world)

This is easy enough for us to do, since we've already got a dispatch=... interface, so we'll want something like this:

if app is not None:
    if is_coroutine(app):
        dispatch = ASGIDispatcher(app)
    else:
        dispatch = WSGIDispatcher(app)

We'll then need ASGIDispatcher and WSGIDispatcher implementations, which implement Dispatcher.send().