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/streams/flow_control.rs

@@ -1,6 +1,6 @@
-use ConnectionError;
+use frame::Reason;
+use frame::Reason::*;
 use proto::*;
-use error::Reason::*;
 
 #[derive(Copy, Clone, Debug)]
 pub struct FlowControl {
@@ -67,15 +67,15 @@ impl FlowControl {
     /// Increase the window size.
     ///
     /// This is called after receiving a WINDOW_UPDATE frame
-    pub fn inc_window(&mut self, sz: WindowSize) -> Result<(), ConnectionError> {
+    pub fn inc_window(&mut self, sz: WindowSize) -> Result<(), Reason> {
         let (val, overflow) = self.window_size.overflowing_add(sz as i32);
 
         if overflow {
-            return Err(FlowControlError.into());
+            return Err(FlowControlError);
         }
 
         if val > MAX_WINDOW_SIZE as i32 {
-            return Err(FlowControlError.into());
+            return Err(FlowControlError);
         }
 
         trace!("inc_window; sz={}; old={}; new={}", sz, self.window_size, val);