Reimport mustache into server module

Rust's module and crate system is kinda confusing for me (comming from Ruby land).
But I think I found a inconsistence.

In https://github.com/nickel-org/nickel.rs/blob/master/src/response.rs mustache is made available for this module scope via ```use mustache```. I think the server module should use the same approach.

Correct me if I got something wrong.