A special type of actor that adheres to a parent grid.
For a GridActor to work correctly, the actor passed as the base parameter needs to expose the following:
_cells[col][row] == cell at (col,row)
base:Another Actor upon which to 'base' this one (in the scene graph). A properly implemented jStage game will use a jStage.Stage as the base of the scene graph, so this parameter should not be null or undefined.