Stub out window update

src/frame/window_update.rs

@@ -0,0 +1,28 @@
+use frame::{Head, Error};
+use super::{StreamId};
+
+#[derive(Debug)]
+pub struct WindowUpdate {
+    stream_id: StreamId,
+    size_increment: u32,
+}
+
+const SIZE_INCREMENT_MASK: u32 = 1 << 31;
+
+impl WindowUpdate {
+    pub fn load(head: Head, payload: &[u8]) -> Result<WindowUpdate, Error> {
+        if payload.len() != 4 {
+            // Invalid payload len
+            // TODO: Handle error
+            unimplemented!();
+        }
+
+        let size_increment = unpack_octets_4!(payload, 0, u32)
+            & !SIZE_INCREMENT_MASK;
+
+        Ok(WindowUpdate {
+            stream_id: head.stream_id(),
+            size_increment: size_increment,
+        })
+    }
+}