[WIP] Expand on Entities API

This PR represents the progress in adding all entities to Sponge with methods for functionality. Ideally this will mirror MC Source, Forge, and Bukkit API in a general abstract functionality.

__Entities__
- [ ] setSize(), getXZSize(), getHeight()
- [ ] getScale(), setScale()
- [ ] getMaxHealth(), setMaxHealth()

__Merchants__
- [ ] getCustomer(), setCustomer()

__Tameable__
- [ ] isTamed(), setTamed()
- [ ] getOwner(), setOwner()

__Tamer__
- [ ] getName()
- [ ] getUniqueId()

__Ageable__
- [ ] getAge(), setAge()
- [ ] isBaby(), setBaby()
- [ ] isAdult(), setAdult()
- [ ] canBreed(), setBreeding()
- [ ] setScaleForAge()

__Villager__
- [ ] getProfession(), setProfession()
- [ ] isPlaying()
- [ ] isTrading()

__Bat__
- [ ] isAwake(), setAwake()

__Skeleton__
- [ ] getSkeletonType(), setSkeletonType()

__Slime__
- [ ] getSize(), setSize()