Integrate TicTacToe and use endpoints.CurrentUser.

- imports tictactoe package from https://github.com/crhym3/go-tictactoe/tictactoe;
- uses endpoints.CurrentUser to get the user associated with a request.

TicTacToe game will be available at http://host/tictactoe URL. Live version is already being hosted at https://go-endpoints.appspot.com/tictactoe

**NOTE**: wait to merge this with master 'till https://github.com/crhym3/go-endpoints/issues/2 and https://github.com/crhym3/go-endpoints/issues/6 are closed and merged with their master.