Ref count stream state and release when final (#73)

Previously, stream state was never released so that long-lived connections
leaked memory.

Now, stream states are reference-counted and freed from the stream slab
when complete.  Locally reset streams are retained so that received frames
may be ignored.

src/lib.rs

@@ -24,6 +24,8 @@ extern crate log;
 
 extern crate string;
 
+extern crate ordermap;
+
 mod error;
 mod codec;
 mod hpack;