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

tests/support/frames.rs

@@ -249,6 +249,11 @@ impl Mock<frame::Reset> {
         let id = self.0.stream_id();
         Mock(frame::Reset::new(id, frame::Reason::FLOW_CONTROL_ERROR))
     }
+
+    pub fn refused(self) -> Self {
+        let id = self.0.stream_id();
+        Mock(frame::Reset::new(id, frame::Reason::REFUSED_STREAM))
+    }
 }
 
 impl From<Mock<frame::Reset>> for SendFrame {