Proposed addition to expose isForward

binding.js

  /**
    Returns the direction of the binding.  If isForward is YES, then the value 
    being passed came from the "from" side of the binding (i.e. the "Binding.path" 
    you named).  If isForward is NO, then the value came from the "to" side (i.e. 
    the property you named with "propertyBinding").  You can vary your transform 
    behavior if you are based on the direction of the change.
    
    @returns {Boolean}
  */
  isForward: function() {
    return this._fromTarget === this._bindingSource;
  }