input: Write a python function to count occurrences of a substring within a string.
  assert count_occurences('aa', 'aaaaaa') == 5
  assert count_occurences('ab', 'abababab') == 4
  assert count_occurences('abc', 'abcabcabc') == 3
output: def count_occurences(substring, string):
      count = start = 0
      while start < len(string):
          start = string.find(substring, start) + 1
          if start > 0:
              count += 1
          else:
              break
      return count