Add methods to {client, server}::Builder to set max concurrent streams (#150)

This PR adds `max_concurrent_streams()` methods to the client and server `Builder`s to set the `max_concurrent_streams` setting. I've added unit tests to ensure the correct SETTINGS frame is sent.

Closes #106

src/proto/connection.rs

@@ -76,7 +76,9 @@ where
             local_next_stream_id: next_stream_id,
             local_push_enabled: settings.is_push_enabled(),
             remote_init_window_sz: DEFAULT_INITIAL_WINDOW_SIZE,
-            remote_max_initiated: None,
+            remote_max_initiated: settings
+                .max_concurrent_streams()
+                .map(|max| max as usize),
         });
         Connection {
             state: State::Open,