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/server.rs

@@ -1,5 +1,5 @@
 extern crate h2_test_support;
-use h2_test_support::*;
+use h2_test_support::prelude::*;
 
 const SETTINGS: &'static [u8] = &[0, 0, 0, 4, 0, 0, 0, 0, 0];
 const SETTINGS_ACK: &'static [u8] = &[0, 0, 0, 4, 1, 0, 0, 0, 0];