Game basics working!

Added functionality:
 - Ball speed depends on the river width
 - A "goal" wall can be set up, so that when the player crosses it, he
wins.
 - Health is disminished after a collision with an obstacle/enemy. If
player reaches 0 (or negative) health points, the game ends. The
player's color turns into red as his health is worse.
 - Pause functionality added ("Escape" key). Even the background music
stops.
 - Pirates of the Caribbean soundtrack used as game's level 1 theme.
I think that's all for the moment ;)