Use more concrete examples

There's a lot of ambiguity in the examples that slowed me down in understanding what exactly was going on. I think if the examples were fully functional, it would help a lot in quickly learning what this library is capable of.

For example, readFile here is left to the users' imagination. A concrete, working example that I can use out of the box would be more helpful:

```
var data = _(filenames).map(readFile).parallel(4);
```