Write a class for a referral system.

methods:
addReferral(emp1, emp2): emp1 refers emp2
findReferral(emp): employees referred by emp
findKthDistanceReferrals(emp, k): employees referred by emp with distance k

sample network:
emp1 -> emp2, emp3 (emp1 refers emp2 and emp3)
emp2 -> emp4, emp5
emp3 -> emp6

output for findKthDistanceReferrals(emp1, k = 2):
emp4, emp5, emp6