Add damage modifier; adjust some damage values

Normally, the damage values represent how much damage will be taken a
second (if you remain in a flame, for example). But since the short-term
invincibility given after taking a hit, the damage taken is
insignificant. You only get a split-second of the total damage.

I added a damage modifier to increase the damage taken by the player. I
also adjusted the damage values to provide some challenge.

I didn't change the core system of how the damage is applied because
other objects on the map still use the damage-per-second system, like
the trees, for example.