feat(examples): add an example server with a custom event loop

[mwanner]

Not sure if this example is really worth it. Follows the approach outlined in #1075.

• [X ] The commit messages match the guidelines in https://github.com/hyperium/hyper/blob/master/CONTRIBUTING.md#git-commit-guidelines

[izolyomi]

Being new to Rust and Hyper, this example just have saved me from a lot more of investigation. It was just what I needed, so it's absolutely useful, something like this should be between the official examples.

[seanmonstar]

Sorry for not noticing a while ago!

I'm trying to have any and all examples be in the form of guides, as opposed to in the source repo. So I'm going to close this, but the guides definitely could use an "advanced" server guide showing how to use your own Core.