Show watcher.start in README and remove old (unused) threadStarted var

#19 explained the need for the README update.

#21 Has been fixed [here](https://github.com/pipobscure/fsevents/commit/7dcdf9fa3f8956610fd6f69f72c67bace2de7138#diff-74c86881c60b89f8c61a6d95876b4f72L7).  We removed the declaration to avoid confusion later.  The events not firing are the result of threadStarted being false and a premature return.  

Could you please push `0.2.1` or a new version as it resolves our issues.  Thanks in advance!

Cheers