feat(bsSelect): enable validation on the selectpicker
fix(select): part 2, bind on required attribute changes

The `bsSelect` element should handle its own validation CSS classes by deriving
them from the original select `element`.
The validation is checked when one of the following changes: `$modelValue`, `$viewValue`, `attrs.required`.
Merge pull request #270 from frapontillo/feature/select-validation

fix(select): validate the select element if required on the original one