Add mount! macro to simplify the construction of Mount objects

[jpmartin2]

I've been doing some work on a simple web application in rust using iron, and while creating the routes for it, I noticed that the router crate had a convenient macro to create the router handler, but mount doesn't. I wanted to be able to write

let handler = mount! {
    "/static" => Static::new(Path::new("path/to/static/files")),
    "/" => router! {
        get "/" => some_other_hanler,
        ...
    },
};

This PR adds the mount! macro so you can write that instead of

let router = router {
    get "/" => some_other_handler,
   ...
};

let mut handler = Mount::new();
handler.mount("/static", Static::new(Path::new("path/to/static/files"));
handler.mount("/", router);

[joshmarlow]

@iron-bot any thoughts on this?

[untitaker]

Hmm, this example suggests that mount can deal with variables in paths when it only matches by prefix. Also your macro example says router!, not mount!.