Added a snake body array and functions for adding body part to the first position and poping the last

The array holds body part objects which consists of x and y coordinates of the body part.
This will be useful later when I need to know if the snake has hit itself, which will end the game.
Changed the function removeSnakeTail for it to remove the last body part of the snake. Commented out a lot of previous remove-tail-code

Made removeSnakeTail find the first .snakeHead-object in the DOM to remove it (because it's always the last part of the snake).
Removed old outcommented remove-tail-code and commented the new code
Pressing a-button on keyboard will add one bodypart to the snake

For debugging purposes. It just adds to the variable snakeLengthLimit, which is allways checked each interval (movement).
Visually it appears as if the snake pauses shortly and gets a longer tail - which is exactly what was desired.