Slider value validation, fixes #904

This fixes #904. There is an issue where Slider accepts non-numeric values to be set, which is a mistake. You can test the weirdness of non-numeric values here: http://yuilibrary.com/yui/docs/slider/slider-basic.html

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As for the `on` to `after` fix: @lsmith caught this. `xSlider.on('valueChange', …)` is present in the inline description, but is `xSlider.after('valueChange', …)` in the full code listing. It should've always been the latter.