Fix web worker setup

[freezy]

This allows bundling the web worker in third-party clients by passing a worker instance to the initialize function of the WPC API.

A Webpack setup would look like this:

1. Install worker-loader: npm install worker-loader --save-dev
2. Import and instantiate the worker:

   import Worker from 'worker-loader!wpc-emu/lib/webclient/webworker.js';
   const wpcEmuWebWorkerApi = WpcEmuWebWorkerApi.initialiseWebworkerAPI(new Worker());
   // wpc api available!

Note that you don't need a separate entry point for the worker when building with Webpack anymore.

This patch is backwards-compatible.

[neophob]

thanks! that resolves one of my quirks!