support groups in createBackgroundTransaction stub

The full api for [createBackgroundTransaction](https://github.com/newrelic/node-newrelic/blob/master/api.js#L433) supports three arguments with the second argument being optional. The fallback stub only supported the *name* and *callback* arguments. I've update the stub and the tests to support the optional *group* argument as the full version of the api does.

Thanks!