Use trait objects and dynamic dispatch to abstract over NetworkStream

Server and client benchmarks show that this makes very little difference
in performance and using dynamic dispatch here is significantly more ergonomic.

This also bounds NetworkStream with Send to prevent incorrect implementations.

Allows the implementation of mock streams for testing and flexibility.

Fixes #5

examples/concurrent-server.rs

@@ -13,7 +13,7 @@ use hyper::header::common::ContentLength;
 use hyper::net::{HttpStream, HttpAcceptor};
 
 trait ConcurrentHandler: Send + Sync {
-    fn handle(&self, req: Request, res: Response<Fresh, HttpStream>);
+    fn handle(&self, req: Request, res: Response<Fresh>);
 }
 
 struct Concurrent<H: ConcurrentHandler> { handler: Arc<H> }
@@ -39,7 +39,7 @@ macro_rules! try_abort(
 struct Echo;
 
 impl ConcurrentHandler for Echo {
-    fn handle(&self, mut req: Request, mut res: Response<Fresh, HttpStream>) {
+    fn handle(&self, mut req: Request, mut res: Response<Fresh>) {
         match req.uri {
             hyper::uri::AbsolutePath(ref path) => match (&req.method, path.as_slice()) {
                 (&Get, "/") | (&Get, "/echo") => {