Add mobile first property to directive attributes

[ddsilva]

Responsive settings use mobile first calculation.

<slick mobile-first="true" responsive="responsiveSettings">
  ...
</slick>

$scope.responsiveSettings = [{
  breakpoint: 300, 
  settings: {
    slidesToShow: 2
  }
},{
  breakpoint: 780,
  settings: 'unslick'
}];