recursive _.pluck() when passed multiple keys

_.pluck() calls itself recursively when passed multiple key arguments. Like so

```javascript
var orders = [
    {orderId: 123, shipping: {method: 'express', cost: 995} },
    {orderId: 124, shipping: {method: 'standard', cost: 495} }
];
_.pluck(orders, 'shipping', 'cost')
=> [995, 495]
```

(branch with updated doco: https://github.com/joelplane/underscore/tree/pluck-take-multiple-args-with-doco)