[GEOS-5906] Gathering attribute statistics in the CSS demo page should be optional

Here is an example, as you can see, I've added type and sample value (from the first feature in the dataset) and allowed to compute min and max on demand

![selezione_061](https://f.cloud.github.com/assets/325433/835454/db26e164-f2db-11e2-9f94-9cc0f0a58861.png)