Consider removing expose(), call() and kill() ?

``` javascript
worker.expose = methodName => {
worker[i] = function() {
return worker.call(methodName, [].slice.call(arguments));
};
};
```
Instead `methodName` parameter should be used:
``` javascript
worker.expose = methodName => {
worker[methodName] = function() {
return worker.call(methodName, [].slice.call(arguments));
};
};
```
Beside that I don't see why the `call` and `expose` methods are accessible from outside. (Don't assign to worker at all)
The purpose of the `kill` method is also not clear for me.