worker starting further workers

I recently ran into a bad trap: I had another `require('web-worker')` in the code of the child and not only in the parent. This doesn't work, because the `workerThread` function is then executed again. The symptom is that messages are received twice in the worker.

This was hard to figure out. Note that this is not really a user's error, because a web worker is normally allowed to start another web worker. So it makes sense in general. (In my code, I could simply comment the bad `require` out, though.)

I guess that you need to split `node.js` into two files for this, because the main/worker dichotomy doesn't work anymore when workers can start workers.