Winnow results unless search value is empty

@pfiller When clicking on a Chosen select, it is calling `winnow_results()` even when there is nothing in the search field. Adding the conditional greatly improves performance.

I used `console.time()` to test the performance (randomly opening/closing selects on the demo page)

![screen shot 2013-05-20 at 3 35 03 pm](https://f.cloud.github.com/assets/127289/538510/3dd7af80-c18d-11e2-8c60-43bee852ca25.png)