Overhaul the stat system and implement chaining

- Add Battle.prototype.chain to chain modifiers
- BattlePokemon.prototype.getStat is passed different parameters and no longer runs the ModifyBoost event nor the ModifyStat events.
Overhaul the stat system and implement chaining

onModifyStat events now alter the returning modifier instead of
affecting the stat directly
Run ModifyDef/SpD events properly

and remove Chlorophyll's debug message
Undo scripts.js regression
Re-add getStat function
Merge pull request #336 from V4Victini/stat-overhaul

Stat overhaul