Added some dummy echoing server, just to practice.

To test it, just run ``make run-server'' from the root of the repo.
Currently it always works on port 9992.

Probably the most easy way to test it is to open the address
127.0.0.1:9992 in your favorite browser; it should print the text of the
GET request it sent to my server (and got it back).

Or, of course, you can run something like ``telnet localhost 9992'' and
enjoy the fun of truly interactive process of testing a dummy program.