Added adding classes per node. Auto add leaf class to leaves.

Solves Issue #40. Makes it easier to style specific nodes. Instructions added to the README: 

If you would like to add classes to a certain node, give it an array of classes like so:
  
    $scope.my_data = [{
      label: 'Languages',
      children: ['Jade','Less','Coffeescript']
      classes: ["special", "red"]
    }]

Each element without children, or leaf, is automatically given a leaf class.