Valid `value` to ensure it is strictly numeric.
Addition of unit tests to ensure `Slider` only accepts numeric values.
Correction of `on` to `after`.
Removal of `validator` logic in setter.
Addition of validation logic in `_setNewValue`.
Merge branch 'slider-validation-fix' of github.com:ezequiel/yui3 into ezequiel-slider-validation-fix
Add slider build files.