New feature: ok() without callback

This PR adds the possibility to use okay without any arguments which is useful in situations where you don't need to wait for the result:
```js
fs.writeFile(path, 'hello', ok());
```

It also fixes the `keywords` field in package.json which currently results in 
`"d, o, m, a, i, n, d, o, m, a, i, n, s"` being displayed on the npm site.

I also split up the example into an annotated one and one without comments so that it becomes more obvious that the variant with okay is actually shorter ;)