code: function( behavior ){
  
              var notify;
  
              // don't allow duplicates
              if ( this.has( behavior ) ){
                  return this;
              }
  
              behavior.setWorld( this );
              this._behaviors.push( behavior );
  
              this.emit( 'add:behavior', {
                  behavior: behavior
              });
  
              return this;
          }
docs: chainable
  Physics.world#addBehavior( behavior ) -> this
  - behavior (Behavior): The behavior to add
  
  Add a behavior to the world