Defaulting `options.cwd` to `process.cwd()`, passing `options.cwd` to `worker.js`, fixing `worker.js` by setting `global.__dirname` to `obj.cwd` on init, really fixes #10

`kill("SIGHUP")` does not work on Windows:

Error: kill ENOSYS
    at exports._errnoException (util.js:870:11)
    at ChildProcess.kill (internal/child_process.js:374:13)
    at Worker.terminate (C:\Users\xkr47\project\node_modules\tiny-worker\lib\index.js:55:15)

SIGINT works however, and fix confirmed elsewhere also: https://github.com/balderdashy/sails/issues/846#issuecomment-25187082

Adding `require` to the inline script context for convenience, fixing API mistake by removing `close()` from the instance & updating README