Implement ping_pong (#1)

* comments

* wip

* wip

* Sketch out pingpong and keepalive stack modules

PingPong responds to ping requests with acknowledgements.
KeepAlive issues ping requests on idle connections.

* remove keepalive for now

* commentary

* prettify ping_pong's poll

* test ping pong and passthrough

* add buffering test

* Use a fixed-size slice for ping payloads

* Improve pong dispatch

pong messages should be buffered until Stream::poll returns
Async::NotReady or Async::Ready(None) (i.e. such that it is not expected
to be polled again).  pong messages are now dispatched when no more data
may be polled or the Sink half is activated.

* revert name change

* touchup

* wip

* Simplify Stream::poll

Now PingPong only holds at most one pending pong and the stream will not
produce additional frames unti the ping has been sent.

Furthermore, we shouldn't have to call inner.poll_complete quite so
frequently.

* avoid Bytes::split_to

* only use buf internally to Ping::load

src/proto/mod.rs

@@ -46,7 +46,7 @@ pub fn from_io<T, P>(io: T, settings: frame::SettingSet)
     // Map to `Frame` types
     let framed = FramedRead::new(framed_read);
 
-    // Add ping/pong handler
+    // Add ping/pong responder.
     let ping_pong = PingPong::new(framed);
 
     // Add settings handler