Added DirtMeterUtilityTEST to test out the DirtMeterUtility class - everything tests out okay.  Added a minor changes to DirtMeterUtility - made getCurrentDirtCapacity() to public pretty much for testing purposes, and added a condition to cleanOneDirtUnitFromTile so that it doesn't increment dirt capacity and decrement dirt on tile when tile's dirt amount is already equal to 0 (so that it doesn't clean a clean tile)