Added potions and entity effect accessors

Don't merge yet - opening PR for public discussion and suggestions.
Fixes #922, #942, #899 and I believe #926.

**What I've done**
- Entity effect map in `cPawn`
- Removed food-poisoning-specific code in `cPlayer`, instead opting for the centralized entity effect approach
- Implemented some entity effects
- Implemented milk
- Implemented drinkable potions
- Implemented splash potions (affect players and mobs)
- Fixed issue preventing players from instantaneously healing
- Cave spider poisons its victim

**What I have yet to do**
- All of the many TODO's I've left
  - Implement the rest of the entity effects
  - Wither skull implementation
  - Better potion splash effectiveness equation
- Fix splash potion color when thrown

**What I need help with**
- API hooks
- World storage for entity effects and potions

~~Note: proper potion (especially splash potion) functionality depends on #1078.~~

Feel free to contribute at the `potions` branch. 
Is there some kind of Entity ID we can use?