test grid is returned
find value at given board coordinate
added test for finding board coordinate value
set the value of a board coordinate
added test for set_cell method
add game_over method to asses grid and return result
add tests for game_over method
added core_extensions.rb
wrote tests for #all_empty?
added all_same? method
wrote tests for all_same? method
wrote any_empty? method
added none_empty? method
added any_empty? method
wrote tests for non_empty? method
wrote draw and winner methods
wrote winner and draw tests
added missing winning_position_values method
added Game class
added Game initialize tests