Removes narwhal scripts, fixes #224
Fix examples.

No more ringo/scheduler module, no more pre-imported system module.
Re-add ability of getCurrentWorker() to get worker from scope argument.

This also makes getCurrentWorker stricter by throwing an exception if no worker
is associated with the current thread or the argument object, or if different
workers are associated with both.
Add examples/websocket-server-push.js
fixed --force option

don't close ZipFile if package directory exists and --force has been specified, otherwise
the installation fails with a "zip file closed" exception