Some unit test speed-ups; Removed CellVM.Brush property
Implemented INotifyPropertyChanging in Map
Throw ArgumentNullException when attempting to set map Start or Goal properties to null
Bind MapView's StartCell and GoalCell to MapVM instead of directly to Map
Bind MapView's RowCount and ColumnCount to MapVM instead of directly to Map
Marked Map.PropertyChanging property non-serialized