Gameplay update

I adjusted some features in the gameplay:

• I changed the leveling formula, from now on the exp you need to level equals:
  level * level - (2 * level - 1)   (mathematicians will get it)
• From now on you will start with a small amount of fish being able to spawn in your game (5 at the moment). The higher your level, the more fish will spawn.
• Not every kind of fish will spawn from level 1. Every x levels (currently 3) you get, a new kind of fish will be added to the possibilities to spawn and/or be the fish you need to eat.