Fire event: "time-selected" + removed unused attrs

- When user press the done button, <md-timepicker> fires a
"time-selected" event.
e.detail.time contain a String on the form "HH:mm"
- Removed attributes "twentyfour" and "active" because not used for now
- Simplified "colors" to "color". That one color is passed as a string
and act of the display background-color.