Reworked Grid initializer to properly initialize with Null objects

This commit is being made due to a bug with the initializer of the Grid class. Previously the two dimensional arraywould initWithCapacity but that did not garuntee that when an object was added, it would be placed into correct index. Instead the object would be placed to the first available index (as defined by the NSMutableArray class referece) instead of the correct index.

Now, the grid is initialized with all indecies filled with NSNull objects. Therefore when an object is added at an index it replaces the NSNull object and keeps its correct position because there are NSNull objects in the indecies infront of it.