Added potions and entity effect accessors

Don't merge yet - opening PR for public discussion and suggestions.
Fixes #922, #942 

**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
- Fixed issue preventing players from instantaneously healing

**What I have yet to do**
- All of the many TODO's I've left
  - Implement the rest of the entity effects
  - Splash potion splash on non-player pawns
  - Wither skull implementation
- Splash potions do not appear physically

**What I need help with**
- ToLua (e.g. what should be exposed to it)
- World storage for entity effects

~~Note: proper potion (especially splash potion) functionality depends on #1078.~~

Feel free to contribute at the `potions` branch. 
I'm not too fond of the name "User". On the other hand, I can't come up with a better name...