Reuse worker

Fixes #73 

Been testing this with the testsuites in QUnit itself and jQuery UI. It launches just one worker per browser and reuses them as expected.

In addition to fixing various bugs in @dhimil's implementation, I've also made a useful improvement, by always setting `config.build`. The format is based on what Karma uses for local runs.

@dhimil's commit introduced a `reuseWorker` configuration option. I didn't see the point of making this configurable, so removed the option. If there's more than one test page, workers are automatically reused.