Changing expModifier and hpModifier algo to be less influenced by level....

... For high lvls (~50), you actually get HP instead of losing them with the current formula. This now doesn't happen anymore (at least for levels up to 100, which is the current max).
Also changed the formula in the same way for the exp modifier, although there it didn't create any problems. But I thought it's better to have the influence of the level be the same everywhere.