Timer added and some code cleanup

There is a timer now, if the time is up, the level restarts. 

I also cleaned up the code quite a bit :)

After finishing the level you will not get back to the main screen, but the level will be restarted - so you can get even higher scores :)

Next tasks:

- create more levels and implement level progression system

- when level is finished implement a screen that calculates new score (depending on time left and a finishing bonus)

- more enemies with new behaviour

- ELEVATORS!!!