Add problem 38 and 39 description

Working on bat damage taking and visualising it
Add DamageText to FirstCaveBat class

DamageText now is drawing and updating in FirstCaveBat class. But we
need to increase damage number with subsequent hits.
Refine damage text looking

Damage Text now has behaviour more similar to the original:
- on first hit it floats upward showing '-1'
- subsequent hits decreases number by '-1' and stops floating
- text disappears after 2 seconds after the last hit