Added simple initiation of TicTacToeGrid
Added new class TicTacToeCell and created grid from objects of this class
Added Grid and Cell widgets description to TicTacToe.kv file
Fixed TicTacToe.kv file syntax
Fixed TicTacToeApp class so that its build method returns valid object and changed the way parameters are given in TicTacToeGrid __init__ for creating cell object