make from_str() consistent for United States

[mplanchard]

All of the from_str() alias match arms except for the United States use the alias stripped of any spaces, so you can pass in your country name like, for example:

let input = "Great Britain";
let country = Country::from_str(input.replace(" ", "").as_str()).unwrap();

However, the "united states" alias added in #1 breaks expectations since it contains a space, while no other country aliases contain spaces, and so requires doing something weird for the above example like:

let input = "United States";
let country_input = match input.to_lowercase().as_str() {
    "united states" => input,
    _ => input.replace(" ", "")
};
let country = Country::from_str(country_input.as_str()).unwrap();

This PR adds a new "unitedstates" alias to bring the US in line with the rest of the countries in the from_str implementation. The existing "united states" alias is not removed to avoid breaking backwards compatibility.

I also updated the docs a little bit to make it more obvious that users should strip spaces out of the input before passing to the various methods, both by stating it explicitly and by including examples like "UnitedStates" in the from_str() doctests, which have also been slightly updated to perform assertions in addition to assigning values.