feat(slider): remove the ui-slider-track tag
style(slider): fix indentation
feat(slider): use :after to display the track

the thumb still goes form 0% to 100% so, like `left : 100%` is out of the parent box, we have to reduce the sliding area a bit. It's tricky but it works.
feat(slider): remove  the explicit ui-slider-track

Following https://github.com/angular-ui/ui-slider/pull/29#issuecomment-29855813
chore(dep): update to angular 1.2.4