Initial implementation of interceptors.

I'm anticipating implementing interceptors at two independent parts
of the API. The first is high-level interceptors, that intercept
before the socket connection has been established.

The second set of interceptors will only execute for requests that
require a socket connection. Those are forthcoming.
Use one OkHttp interceptor for each request or response interceptor. If you have 2 request interceptors and 3 response interceptors, you should have 5 OkHttp interceptors.