Write a JavaScript function which takes an array of integers as a parameter and returns the
average of all even integers in the array.
For example, a call of evenAverage([1, 2, 3, 4, 8, 7, 6, 5]) should return 5
(since only the four values 2, 4, 6, and 8 are considered in the calculation of the average result) and
evenAverage([5, 4, 3]) should return 4 since 4 is the only even integer in the array.
If the array passed is empty or does not contain any even integers, return 0. You may assume the array
passed is non-null.
