Mysql connection pool middleware for Iron web framework

This middleware will insert connection pool into request.extensions

Usage:

```rust

extern crate iron;
extern crate mysql;
extern crate iron_middleware_mysql;

use iron::prelude::*;
use mysql::conn::pool::Pool;
use iron_middleware_mysql::DBPool;


fn hello_world(request: &mut Request) -> IronResult<Response> {
    let db_pool = request.extensions.get::<DBPool>().unwrap();
    let res = db_pool.prep_exec("select 23", ()).unwrap().next().unwrap().unwrap().unwrap()[0].clone();
    Ok(Response::with((iron::status::Ok, format!("Got {:?}", res))))
}


fn main() {

    let pool = Pool::new("mysql://user:password@localhost:3306").unwrap();

    let dbpool = DBPool{pool: pool.clone()};
    let mut chain = Chain::new(hello_world);
    chain.link_before(dbpool);
    Iron::new(chain).http("localhost:3000").unwrap();
}

```

See included example for details