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/error.rs

@@ -0,0 +1,34 @@
+use frame::Reason;
+use codec::RecvError;
+
+use std::io;
+
+/// Either an H2 reason  or an I/O error
+#[derive(Debug)]
+pub enum Error {
+    Proto(Reason),
+    Io(io::Error),
+}
+
+impl Error {
+    pub fn into_connection_recv_error(self) -> RecvError {
+        use self::Error::*;
+
+        match self {
+            Proto(reason) => RecvError::Connection(reason),
+            Io(e) => RecvError::Io(e),
+        }
+    }
+}
+
+impl From<Reason> for Error {
+    fn from(src: Reason) -> Self {
+        Error::Proto(src)
+    }
+}
+
+impl From<io::Error> for Error {
+    fn from(src: io::Error) -> Self {
+        Error::Io(src)
+    }
+}