Added example with echo/discard/chargen services.

Create an examples/ directory containing sample code for a unikernel with simple echo/discard/chargen TCP services running on ports 7, 9, and 19.
subjective style: the `>>` operator is actually a camlp4 extension that expands to `>>= fun () ->`

since camlp4 is slowly being deprecated, we've started to prefer not using `>>` in new code, and just doing the "raw" version instead (`>>= fun () ->`) even though it's more verbose.