fixes for fleet 0.8.3 & added tunnel opt

Here are some quick changes I've made to make sure this lib works with fleetctl 0.8.3.

* list-units: no longer returns 7 columns. It now returns 4.
* stop: opt -noblock changed to -no-block

New features:

* list-unit-files
* tunnel support (passed via options) I havent updated README.

```
var fleetctl = new Fleetctl({
  binary: "/usr/local/bin/fleetctl"
  tunnel: '127.0.0.1'
});
```