Expose javascript API

It'd be nice if there was an API exported from the javascript version.

I'd like to just do something like this to start the server in my own application:

```
var websockify = require('websockify');

websockify.start({
   destinationPort: 4567,
   sourcePort: 1234
});
```

In es5 notation, you can just do something like:

`module.exports = someObjectToExpose`
