Add problem 21 description (player/enemy collision)
Constantionalize everything

Every method which can be constant is constant now; every method
argument too.
Implement colision detection with enemy

When Quote collides with enemy method collidesWith() of collision
rectangles returns true.
Add problem 22 description (reaction to damage)
Implement player jump on taking damage

When player takes damage we call method takeDamage() which makes player
tile to do small jump.
Make player sprite flashing on damage