docs(example): explicit tokio runtimes in examples

[dswij]

closes #3448

This PR updates the examples with the implementation of TokioIo and TokioTimer in the file itself.

I guess there's a point from the issue that one needs to look through some files to get a working example. While the examples themselves are fine, and the hyper getting started docs points to hyper-util, we should be more verbose in the examples to explicitly show what hyper will look like.