Use endpoints.CurrentUser method to get the user as associated with a request

This will also split up TicTacToeApi service and its initialization during startup, which would allow to use/integrate `tictactoe` package in another app, e.g.
https://github.com/crhym3/go-endpoints.appspot.com

**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 master.