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).

examples/client.rs

@@ -24,9 +24,9 @@ struct Process {
 
 impl Future for Process {
     type Item = ();
-    type Error = ConnectionError;
+    type Error = h2::Error;
 
-    fn poll(&mut self) -> Poll<(), ConnectionError> {
+    fn poll(&mut self) -> Poll<(), h2::Error> {
         loop {
             if self.trailers {
                 let trailers = try_ready!(self.body.poll_trailers());
@@ -71,7 +71,7 @@ pub fn main() {
             .uri("https://http2.akamai.com/")
             .body(()).unwrap();
 
-        let mut trailers = h2::HeaderMap::new();
+        let mut trailers = HeaderMap::new();
         trailers.insert("zomg", "hello".parse().unwrap());
 
         let mut stream = client.request(request, false).unwrap();