Add methods for creating a PUT and DELETE request

[pintariching]

Currently the Request struct only has methods for GET and POST. I think it would make sense to add also the PUT and DELETE. Up to version 0.5.0, I generated them by hand like for example:

let req = ehttp::Request {
    method: "PUT".to_string(),
    url: put_url,,
    body: json,
    headers: ehttp::Headers::new(&[("Content-Type", "application/json")]),
};

In the version 0.6.0 I see two new fields mode and timeout were added. however the ehttp::type module is private and the Request struct can't be constructed outside of the ehttp crate.