code: function toggleClasses( element, state, openingClass, closingClass ) {
  
  		if( state === 'opening' || state === 'open' ) {
  			var oldClass = openingClass || 'au-accordion--closed';
  			var newClass = closingClass || 'au-accordion--open';
  		}
  		else {
  			var oldClass = closingClass || 'au-accordion--open';
  			var newClass = openingClass || 'au-accordion--closed';
  		}
  
  		removeClass( element, oldClass );
  		addClass( element, newClass );
  	}
docs: PRIVATE
  IE8 compatible function for replacing classes on a DOM node
  
  @param  {object} element      - The DOM element we want to toggle classes on
  @param  {object} target       - The DOM element we want to toggle classes on
  @param  {object} state        - The current state of the animation on the element
  @param  {string} openingClass - The firstClass you want to toggle on the DOM node
  @param  {string} closingClass - The secondClass you want to toggle on the DOM node