Forage Balancing

Proposed formula: u.skillLevel("survival") + (u.per_cur / 2 - 4) + 3

Basically, it scales more with survival skill and less with perception than what is currently in and it has lower fixed chance: +3 instead of +5.

Charts:
![chart1](https://cloud.githubusercontent.com/assets/6969268/3795771/3d45894c-1bbf-11e4-827a-07a5e4d1e7b0.PNG)
![chart2](https://cloud.githubusercontent.com/assets/6969268/3795773/3e69d08a-1bbf-11e4-8d93-d19d8e01b427.PNG)

Eggs are spawning so much, because in spring, they are not sharing spawn pool with wild vegetable. This is another area which might need a tweak. Maybe with bigger forage overhaul.