Add _.chunk to Array functions.

Fixes #1891 
I would probably write line 655-659 as

```js
var i = 0, length = array && array.length, j = 0;
while (i < length && count >= 0) {
     if (i % count) result[j++] = [];
     result[j].push(array[i]);
}
```

(untested). I would be curious to see some benchmarks