Scanner Exercise 1

Write a new Java class called Averages that calculates 
the average of a series of user-entered values:
- import the java.util.Scanner class 
- add the code that repeatedly asks the user for positive 
  integer values
- stop when the user enters a negative number or 0
- after all the integers have been read, calculate and 
  display the average of all integers entered

Tips: 
- as each integer is entered, add it to a sum variable
- only calculate the average AFTER all integers have 
  been entered