code: function( origin ){
              var centroid = this.getCentroid();
              var delta = origin !== undefined ? origin.sub( centroid ) : centroid.invert();
              for( var i=0, l = this.vertices.length; i<l; i++){
                  this.vertices[i].addSelf( delta );
              }
              return this;
          }
docs: centers the polygon so that its new centroid is at the given point
  @param {Vec2D} [origin]
  @return itself