Add trait implementation for Serde structs for use as Value

[jacobh]

I'm using this like this...

use iron_sessionstorage;
use uuid::Uuid;

#[derive(Serialize, Deserialize)]
pub struct UserSession {
    session_id: Uuid,
}
impl UserSession {
    pub fn new_with_random_id() -> UserSession {
        UserSession {
            session_id: Uuid::new_v4(),
        }
    }
}
impl iron_sessionstorage::Key for UserSession {
    fn get_key() -> &'static str {
        "user_session"
    }
}

...

pub fn auth_post_handler(req: &mut Request) -> IronResult<Response> {
    let form = req.get::<bodyparser::Struct<AuthForm>>();
    match form {
        Ok(Some(form)) => if form.password == ADMIN_PASSWORD {
            match req.session().get::<UserSession>()? {
                None => {
                    req.session().set(UserSession::new_with_random_id())?;
                }
                _ => {}
            }
            return Ok(Response::with(status::Ok));
        } else {
            return Ok(Response::with(status::Unauthorized));
        },
        _ => return Ok(Response::with(status::BadRequest)),
    }
}