fixed a calculation bug...

After 50 minutes of working on that monstrosity I thought: Why not use
Inno's function..?
That one should work even if new keys like "age of your grandma" are
invented for calculating bonuses...
Merge pull request #5 from petee-d/bugfix-calcItemBonusForJob-and-refactor

replaced faulty function by a call to Inno's Calc