implement callback on write

I believe the intention of highlands `Stream.write` is to conform with the node stream interface. However, it does not take a callback as its third argument, which is causing problems for me in other library code once it is passed a highland stream rather than a "normal" writable stream. In particular there is code like:

```
return new Promise( function(res, rej) {
stream.write(data, null, function (err) { if(!!err) { rej(err); } else { res() }} );
}
```

which breaks when passing a stream created via `_()` around.

Simple to implement by listening to the "drain" event internally if you are emitting that correctly. (Are you interested in a pull request?)

Locate the highland Stream.write implementation used by streams created via _(), then inspect how it emits or handles the drain event. Verify the third callback is invoked with an error when appropriate and without one on success, using the Promise example in the issue as the completion criterion.

Written by the indexing model from the issue text.