Support of pack

`pack` is algorithm able to,

```js
_([1, 2, 3, 4, 5]).pack(2)  // => [1, 2], [3, 4], [5]
```

recently I've been impletementing one [tool](https://github.com/likeastore/elaster) for streaming mongodb collection to elastic search index.. mongo collection is streamed and each record pushed to elastic search. that appeared to be quite slow on big collections. ES provides bulk indexing, which mitigates HTTP overheads a bit. With pack, code will be really nice

```js
var stream = db.items.find({}).sort({_id: -1}); 
_(stream).pack(128). through(elasticBulk);
```

so, `pack` will be useful for any *bulk* stream operations.