Use endpoints.Context and endpoints.CurrentUser().

Additionally:

- Split up TicTacToeApi service and its initialization
  during startup. This will allow to use/integrate tictactoe
  package in another app,
  e.g. http://github.com/crhym3/go-endpoints.appspot.com
- API service methods docs.
- update Cloud Endpoints docs references to point to go-endpoints
  package repo instead of Python version.