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/frame/settings.rs

@@ -74,7 +74,6 @@ impl Settings {
         self.max_concurrent_streams
     }
 
-    #[cfg(feature = "unstable")]
     pub fn set_max_concurrent_streams(&mut self, max: Option<u32>) {
         self.max_concurrent_streams = max;
     }