Added initial PotionTypes and EntityTypes implementation.

In order to avoid duplicate entity names with mods, all entities will be stored in an internal registry as 'modid:name'. If you call EntityType.getByName(id)
and do not include the modid, it will assume 'minecraft:name'.

Potions currently do not have this feature but I plan on adding if needed.

* Added MixIn for EntityRegistry in order to catch mod entities and add
them to our internal registry.
Implemented Living Monsters.
Added license headers.
Implemented Living Animals.

* Implemented Living meta types.
* Implemented ItemStack.
* Updated SpongeAPI.
Implemented rest of Living animals.

* Implemented BiomeTypes.
* Cleaned up some mixin code.
Reorganized mixins to match API package structure.
Fixed wrong name in HorseVariant constant.
Merge pull request #82 from SpongePowered/impl/mcbasetypes

Initial implementation for entities, biomes, items, and potions.