Improve Selector Search for asNavFor and focusOnSelect

Clicking on slides for a slick instance with `asNavFor: true` causes the index lookup to resolve to `undefined` when the `.slick-slide` contains child elements.

This fix continues up through the parents of `event.target` until it finds the DOM node with the `slick-slide` class instead of requiring it be the immediate parent of `event.target`.
Merge pull request #517 from deefour/patch-1

Improve Selector Search for asNavFor and focusOnSelect