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

Cargo.toml

@@ -16,9 +16,6 @@ git = "https://github.com/hyperium/mime.rs"
 [dependencies.unsafe-any]
 git = "https://github.com/reem/rust-unsafe-any"
 
-[dependencies.intertwine]
-git = "https://github.com/reem/rust-intertwine"
-
 [dependencies.move-acceptor]
 git = "https://github.com/reem/rust-move-acceptor"