md-tab colWidth should be responsive

[frankwong15]

As title. We should be able to specify colWidth for different screen size, e.g. s4 m3 l2. Maybe easier to expose colClass directly?

  colWidth: oneWay('composableParent.colWidth'),

  _colClass: computed('colWidth', function() {
    return `s${this.get('colWidth')}`;
  })