adjusting prefetch ttl

This change that allows the ttl for prefetched data to be adjusted by adding `ttl_ms` to a dataset.

```javascript
$('.typeahead').typeahead({                                
  name: 'countries',
  prefetch: '../data/countries.json',
  ttl_ms: 60 * 60 * 1e3, // 1 hour instead of 3 days
});
```