Fix up the "import conflicts" error of server module.

Hi @nickel-org,

I found that `use http::server::{..., Server, ...}` will cause a compiling error which said:

    src/server.rs:3:28: 3:34 error: import conflicts with type in this module
    src/server.rs:3 use http::server::{Config, Server, Request, ResponseWriter};
                                               ^~~~~~
    src/server.rs:31:1: 44:2 note: note conflicting type here

And with the `impl http::server::Server for Server` usage, another error will be raised:

    src/server.rs:43:14: 43:29 error: type `server::Server` does not implement any method in scope named `serve_forever`
    src/server.rs:43         self.serve_forever();

It seems the reason is that only `use path::Trait` can pull the implementations into the current scope, which was mentioned [here](https://github.com/rust-lang/rust/issues/11690).

My patch resolved the two issues and it works with `rustc 0.12.0-pre-nightly (01ec6fab2 2014-08-18 00:46:10 +0000)` and `cargo 0.0.1-pre-nightly (b9c2abf 2014-08-16 19:44:14 +0000)`.