add possibility to add custom css classes

[27leaves]

I needed the possibility to add a custom css class to a slider. With this pull request you can now add

options: {
  className: 'my-custom-class'
}

and it will add it to

<span ng-class="mainSliderClass" id="" class="ng-scope jslider  jslider-single my-custom-class"> ...</span>

[leandro-shifteight]

@darul75 please fix the documentation, it says classname instead of className

[darul75]

Sure