Added a TODO about getting the snake to move
Added a setInterval for "moving" the snake

It only adds a new head to the snake array at the moment, without restrictions, except checking if the new snake heads coordinates already exists
in the snake array - which will result in a game over function.
Added code for removing the tail in each interval

The tail should "grow" out from the first head when the user starts playing, so a constant was added for the start length of the snake
Added updateBoard() and drawGUI() to the interval to make the snake move on the screen
Put the key events in its own function and activated arrow control of the snake
Updated the A-key event with the correct constant name
Added game over function

Put the game over div, start game div and canvas in a wrapper. Fixed the constants to be reset when initializing the game, so that
everything is reset when the game is over.
Changed constant name, gameStarted to isGameStarted for consistency
Removed comments and cleaned up the constants (names and placement)
Added and x:ed TODOs