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.
minor style: no need for brackets here.

```
let message =
  match ... with
  | ... ->
  | ... ->
in
```

(you can use `ocp-indent` to help with this)