feat(net): add socket timeouts to Server and Client

While these methods are marked unstable in libstd, this is behind a
feature flag, `timeouts`. The Client and Server both have
`set_read_timeout` and `set_write_timeout` methods, that will affect all
connections with that entity.

BREAKING CHANGE: Any custom implementation of NetworkStream must now
  implement `set_read_timeout` and `set_write_timeout`, so those will
  break. Most users who only use the provided streams should work with
  no changes needed.

Closes #315

src/http/message.rs

@@ -1,12 +1,16 @@
 //! Defines the `HttpMessage` trait that serves to encapsulate the operations of a single
 //! request-response cycle on any HTTP connection.
 
-use std::fmt::Debug;
 use std::any::{Any, TypeId};
+use std::fmt::Debug;
 use std::io::{Read, Write};
-
 use std::mem;
 
+#[cfg(feature = "timeouts")]
+use std::io;
+#[cfg(feature = "timeouts")]
+use std::time::Duration;
+
 use typeable::Typeable;
 
 use header::Headers;
@@ -62,7 +66,10 @@ pub trait HttpMessage: Write + Read + Send + Any + Typeable + Debug {
     fn get_incoming(&mut self) -> ::Result<ResponseHead>;
     /// Set the read timeout duration for this message.
     #[cfg(feature = "timeouts")]
-    fn set_read_timeout(&self, dur: Option<Duration>) -> ::Result<()>;
+    fn set_read_timeout(&self, dur: Option<Duration>) -> io::Result<()>;
+    /// Set the write timeout duration for this message.
+    #[cfg(feature = "timeouts")]
+    fn set_write_timeout(&self, dur: Option<Duration>) -> io::Result<()>;
     /// Closes the underlying HTTP connection.
     fn close_connection(&mut self) -> ::Result<()>;
 }