Exercise 5
----------

Get familiar with the code of double linked list and the binary tree and test those with the main program.

-To DLlist-class, add a method which can be used to move backwards in the list.

-To binary tree, add a method which tries to find given element(number) in the tree, and if it
was found, it returns the number of elements in the 'subtree' starting from that element. For
example if the tree was:

	6
       / \
      4   8
     /   / \
    2   7   9
   / \       \
  1   3       11

then the method would return:
4, with argument 8 (there is number 8 in the tree and in the 'subtree' there are 7,8,9,11)
2, -   ||   -    9
1, -   ||   -    1
9, -   ||   -    6 (the number of elements in the whole tree)
0, -   ||   -    84 (number 84 doesn't exist in the tree)