added workerUrl as optional parameter

I added workerUrl as an optional parameter, so that it is possible to
compile arbor.js into another minified js file. Previously, arbor looked
for a script tag with arbor.js to get the url for the web worker. If
arbor.js is compiled into a js file with a different name, arbor couldn't load the web worker. With workerUrl you point arbor to the js file it should
use for the web worker. If workerUrl is undefined, it falls back on the
old method of guessing the url.