Thoughts on Sencha Connect middleware.
Use `header` event to stop middleware.

Using the `header` event of the response object, I am able to stop a
Sencha Connect middleware pipeline after a middleware sends headers. All
Sencha Connect middleware can be used from Register. I'd thought that
only pass-through middleware would work with Register, because
only pass-through middleware would affirm it's completion by calling
`next` with or without an error. By waiting on headers, I can also
listen for the other possible outcome of a middleware function; sending
a response to the client.

See #66.