Refactor errors (#46)

This patch does a bunch of refactoring, mostly around error types, but it also
paves the way to allow `Codec` to be used standalone.

* `Codec` (and `FramedRead` / `FramedWrite`) is broken out into a codec module.
* An h2-codec crate is created that re-exports the frame and codec modules.
* New error types are introduced in the internals:
  * `RecvError` represents errors caused by trying to receive a frame.
  * `SendError` represents errors caused by trying to send a frame.
  * `UserError` is an enum of potential errors caused by invalid usage
    by the user of the lib.
  * `ProtoError` is either a `Reason` or an `io::Error`. However it doesn't
    specify connection or stream level.
  * `h2::Error` is an opaque error type and is the only error type exposed
    by the public API (used to be `ConnectionError`).

There are misc code changes to enable this as well. The biggest is a new "sink"
API for `Codec`. It provides buffer which queues up a frame followed by flush
which writes everything that is queued. This departs from the `Sink` trait in
order to provide more accurate error values. For example, buffer can never fail
(but it will panic if `poll_ready` is not called first).

src/proto/ping_pong.rs

@@ -1,7 +1,7 @@
 use frame::Ping;
 use proto::*;
 
-use futures::Sink;
+use std::io;
 
 /// Acknowledges ping requests from the remote.
 #[derive(Debug)]
@@ -45,15 +45,16 @@ impl<B> PingPong<B>
     }
 
     /// Send any pending pongs.
-    pub fn send_pending_pong<T>(&mut self, dst: &mut Codec<T, B>) -> Poll<(), ConnectionError>
+    pub fn send_pending_pong<T>(&mut self, dst: &mut Codec<T, B>) -> Poll<(), io::Error>
         where T: AsyncWrite,
     {
         if let Some(pong) = self.sending_pong.take() {
-            if let AsyncSink::NotReady(pong) = dst.start_send(pong)? {
-                // If the pong can't be sent, save it.
+            if !dst.poll_ready()?.is_ready() {
                 self.sending_pong = Some(pong);
                 return Ok(Async::NotReady);
             }
+
+            dst.buffer(pong).ok().expect("invalid pong frame");
         }
 
         Ok(Async::Ready(()))