Issue 92: More finishing touches
Adds ConditionalInstantiator to instantiate classes that may or may not exist
Move JavaApiPrefabValues to parent package
ConditionalInstantiator returns null if type doesn't exist
Add a PrefabValueBuilder to help instantiate conditionally
Complete PrefabValueBuilder to mirror ConditionalInstantiator
Rename PrefabValueBuilder to ConditionalPrefabValueBuilder