Update the examples for 0.13

[seanmonstar]

Most of the examples/*.rs files still need to be updated to work with std::future::Future. Most can be more easily written using #[tokio::main], like in this example.

[liufuyang]

Can I try update a simple example? Looks like you already got the hello example updated, right?
https://github.com/hyperium/hyper/blob/master/examples/hello.rs

And I should try further update it to use #[tokio::main] ? :)

[liufuyang]

I made a simple change like this: #1852
let me know whether that is the right thing to do :)