Scale mobs exp properly

[Tzook]

As a guideline, the exp a mob gives should be relative to his hp and then modified by his global toughness.
So if a turtle has 10 hp and gives 2 exp, the ratio is 5:1 - therefore we should keep the same 5:1 hp:exp ratio in all mobs. For example, if a bat has 20 hp, his exp would be 4 according to the formula, but he's flying and hard to hit so we'll add 2 more exp.
By this logic, all bosses should give much more exp as they need to apply the hp/exp formula we decided, and increase some exp for their toughness.

@benk0913 what do you think about the 4:1 ratio?