Implement #1283 idea using typed headers

[jespersm]

This PR implements FromRequest for a number of the typed headers from the hyperxcrate.
It depends on PR #1535.

It allows for constructs such as:

use rocket::http::hyper::header::AcceptLanguage;

#[get("/poem")]
fn poem(lang: Option<AcceptLanguage>) -> String {
    if let Some(lang) = lang {
        format!("TODO: Write a poem in '{}'", lang)
    } else {
        format!("A poem in any other language would sound as sweet!")
    }
}

The list of headers may need to be looked at since a number of the supported typed headers are only used in responses.

[SergioBenitez]

I am closing this as an implication of closing #1535.