Add test infrastructure to work directly with frames (#56)

This adds a `Codec` based testing API. This is a bit less annoying than writing
at the raw H2 wire protocol level...

tests/support/src/future_ext.rs

@@ -0,0 +1,32 @@
+use futures::{Future, Poll};
+
+use std::fmt;
+
+pub trait FutureExt: Future {
+    fn unwrap(self) -> Unwrap<Self>
+        where Self: Sized,
+              Self::Error: fmt::Debug,
+    {
+        Unwrap { inner: self }
+    }
+}
+
+pub struct Unwrap<T> {
+    inner: T,
+}
+
+impl<T: Future> FutureExt for T {
+}
+
+impl<T> Future for Unwrap<T>
+    where T: Future,
+          T::Item: fmt::Debug,
+          T::Error: fmt::Debug,
+{
+    type Item = T::Item;
+    type Error = ();
+
+    fn poll(&mut self) -> Poll<T::Item, ()> {
+        Ok(self.inner.poll().unwrap())
+    }
+}