Replace top-level API with `listenTo` equivalents?

[jamesplease]

Per the discussion on #117 and #124, the top-level API might not even be necessary once we have listenTo equivalents.

This is to investigate getting rid of the top-level API once #117 is resolved.

[jamesplease]

I think that the top-level API should stick around. I like it for making one-off requests:

var auth = Radio.request('auth', 'authorized');

listenTo equivalents will be great, but it doesn't take care of all of the use-cases of the top-level API.