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).

util/h2-codec/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "h2-codec"
+version = "0.1.0"
+authors = ["Carl Lerche <me@carllerche.com>"]
+
+[dependencies]
+http = { git = "https://github.com/carllerche/http" }
+log = "0.3.8"
+fnv = "1.0.5"
+bytes = "0.4"
+string = { git = "https://github.com/carllerche/string" }
+byteorder = "1.0"
+futures = "0.1"
+tokio-io = "0.1.3"
+
+# tokio-timer = "0.1"
+# http = { git = "https://github.com/carllerche/http", branch = "lower-case-header-name-parsing" }
+# slab = "0.4.0"