code: function( topic, fn, scope ){
  
              // check if we're subscribing to multiple topics
              // with an object
              if ( Physics.util.isObject( topic ) ){
  
                  for ( var t in topic ){
  
                      this.one( t, topic[ t ], fn, scope );
                  }
  
                  return this;
              }
  
              // set the _one_ flag
              fn._one_ = true;
              this.on( topic, fn, scope );
  
              return this;
          }
docs: Physics.util.pubsub#one( topic, fn( data, event )[, scope, priority] ) -> this
  Physics.util.pubsub#one( topicConfig[, scope, priority] ) -> this
  - topic (String): The topic name
  - topicConfig (Object): A config with key/value pairs of `{ topic: callbackFn, ... }`
  - fn (Function): The callback function (if not using Object as previous argument)
  - data (Mixed): The data sent from the call to `.emit()`
  - event (Object): Event data, holding `.topic`, the topic, and `.handler`, the `fn` callback.
  - scope (Object): The scope to bind callback to
  - priority (Number): The priority of the callback (higher is earlier)
  
  Subscribe callback(s) to a topic(s), but only ONCE.