Update API docs and demos for recent worker improvements.

Changed.

By the way, according to the historical leasons of Java, manually closing resources can be suprisingly hard to get right, so [try-with-resources was introduced](http://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html) finally.

Relying on GC is not ideal, but it is still much easier to get right than manually closing, as long as all browsers implement GC of resources correctly (if this is the case, we can view close() as just an optimization). If you fail to release the worker or the ZipReader object, it is leaked anyway, no matter whether you call close() on it or not.