feat(server): keep-alive!

Internals have been shuffled around such that Request and Reponse are
now given only a mutable reference to the stream, instead of being
allowed to consume it. This allows the server to re-use the streams if
keep-alive is true.

A task pool is used, and the number of the threads can currently be
adjusted by using the `listen_threads()` method on Server.

[breaking-change]

src/header/common/connection.rs

@@ -3,7 +3,7 @@ use std::fmt::{mod, Show};
 use std::str::FromStr;
 use super::util::{from_comma_delimited, fmt_comma_delimited};
 
-use self::ConnectionOption::{KeepAlive, Close, ConnectionHeader};
+pub use self::ConnectionOption::{KeepAlive, Close, ConnectionHeader};
 
 /// The `Connection` header.
 #[deriving(Clone, PartialEq, Show)]