Create PerfectNumbers.java

Check if a number is a perfect number.  That is it is equal to the sum of its proper positive divisors, excluding the number itself. 
It is half the sum of ALL (includes the number itself) of its positive divisors. 
1 + 2 + 3 = 6 is the first perfect number.
Is also half the sum of all its positive divisors: ( 1 + 2 + 3 + 6 ) / 2 = 6.