Add HTTP router

This is the initial PR for discussing #3023 more easily.
For more info, see the commit message.

You can run the example and the tests.

My plan:
* Improve Javadoc of classes.
  Add https://github.com/sinetja/jauter/blob/master/README.rst to
  io.netty.handler.codec.http.routing.package-info.java.
  Add https://github.com/sinetja/netty-router/blob/master/README.rst to
  io.netty.handler.codec.http.router.package-info.java.
* Improve example.
* Improve based on further discussions and feedback.
Return unmodifiable Map ?