added TicTacToe class
added spec for empty board on start and matching code to satisfy spec
added current player spec and code
added spec for #get_random_player
added specs and code to determine if game is over
added additional spec to #over?
added spec and code to determine winner
added code for displaying and marking the board
added game logic for the computer to make the right move
added specs for #computer_mark and #computer_move