input: Write a python function to find the maximum depth of a binary tree.
  assert maxDepth(root) == 3
  assert maxDepth(None) == 0
  root = Node(1)
  root.left = Node(2)
  assert maxDepth(root) == 2
output: class Node:
      def __init__(self, value=0, left=None, right=None):
          self.value = value
          self.left = left
          self.right = right
  
  def maxDepth(root):
      if root is None:
          return 0
      else:
          left_depth = maxDepth(root.left)
          right_depth = maxDepth(root.right)
          return max(left_depth, right_depth) + 1