Potion API

Added a basic Potion API. More methods will be added relating to applying potions once there is an entity implementation.

Additions:

PotionEffect: The effects that potions will apply to entities
PotionType: A type of potion which contains PotionEffect's 
Potion: Takes use of a PotionType and applies it to an entity.

might be a good idea to {@link PotionType}

So have it as "Gets the {@link PotionType} of the potion.' 

Also the First sentence of the java doc should have periods
I mentioned earlier to remove the periods from the  @param and @return

Please fix these throughout your file thanks :)