GameState and Timer

added gamestate and timer to show how much time is left in the game. This can later be restricted to being a timer only, but for now it allows a pause option

Fixes #5

To test:
* go to the GameStateTimerTest scene
* notice that the time left shows up in the corner counting down
* notice how the font changes to red when there is only a minute left

Note: The game time and "red threshold" are all easily configurable in the editor