Provide socket-like API about rustls

[ctz]

I've done this, in two complementary parts:

• complete_io in the Session trait: this function does whatever IO the session currently needs using whatever transport you give it.
• a new Stream type which wraps up a Session and a transport, and implements blocking io::Read and io::Write traits on top of it

The first is intended to be generally useful, the second hopefully solves this bug. Let me know if not.

[Keruspe]

Fwiw, I just published https://docs.rs/rustls-connector/0.6.0/rustls_connector/ which wraps the above in an API similar to the ones from native-tls and openssl.
Not making the version number match rustls yet, until I've done some more testing