Add separate controller action for saving event logs
Previously, logs were saved by sending a POST request to the existing play
action of the GamesController, which rerouted them to the proper function. This
commit adds a separate saveLog action to the GamesController, and sends the
POST requests to that action instead.

Note: actions are functions registered with a controller, which can be accessed
via a URL. For more info, look at the Yii documentation.

Other changes:
 - Store logs in game-specific folders within the analytics folder
   - e.g: StupidRobot logs will be stored in a separate folder than ZenTag logs
   - Note that only StupidRobot has event-logging currently