Make Topic streams pipeable

Probably still a WIP. Useful for instance if you have a sensor emitting data to a nodejs stream

```js
var sensorStream = require("../sensor");

var sensorTopic = ros.Topic({
  name: "ns/sensor"
});
sensorStream.pipe(sensorTopic.toStream());
```

And now we're publishing the sensor stream to "ns/sensor" topic. Easy :)