support for binary websockets

The unopinionated API for using binary sockets (as in this PR) is:

```js
// SERVER
// ...
var stream = wsStream(client, {
  binary: true,
  mask: false
});
// ...
```

```js
// CLIENT
var stream = wsStream('ws://localhost:1337', { binaryType: 'arraybuffer' });
```

The opinionated and more elegant API (not coded yet) could be:

```js
// SERVER
// var stream = wsStream(client, { binary : true });
```

```js
// CLIENT
var stream = wsStream('ws://localhost:1337', { binary: true });
```

What do you think?