Adds support for updating models on dragstop instead of move

@steve8708 and @that70schris, (fyi @amschrader if you're still using this in shopsense)

This fixes the problem with the screen jumping around when you're dragging the slider. If you want to use it, all you have to do is something like:
```
<slider
  values="discountMap"
  ng-model="query.discountPercentageId"
  highlight="right"
  dragstop="true">
</slider>
```