Removes intertwining, since it is slow and showing up in profiles

Intertwining was a nice feature, but it slows down hyper significantly,
so it is being removed.

There is some fallout from this, mainly that Incoming has had its type
parameter changed to `<A = HttpAcceptor>` and Handler receiving one
bounded with `A: NetworkAcceptor`.

[breaking-change]

Fixes #112

src/lib.rs

@@ -134,7 +134,6 @@ extern crate openssl;
 #[cfg(test)] extern crate test;
 extern crate "unsafe-any" as uany;
 extern crate "move-acceptor" as macceptor;
-extern crate intertwine;
 extern crate typeable;
 extern crate cookie;