Support worker.disconnect() and master.fork(name) API. fixes #32

* Support `worker.disconnect()` and `master.fork(name)` API. fixes #32
* add graceful_exit demo codes
* remove `rewire`
* use `jscover` instead of `visionmedia-jscoverage`

`rewire` 非常影响单元测试，会引入一些非预料的bug。