Add find pair list problem

I have added yet another list problem where you have to find pairs of
numbers that sum up to a certain value. I added the solution I came up
with in a coding interview. I think it is pretty poor, since I believe
there is a O(n) solution.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add iterative inorder and postorder traversal

To complete the iterative variants of binary tree traversal this commit
also adds the algorithms for traversing a tree without using recursion.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Update README

I forgot to increment the number of solutions for the iterative binary
tree traversal algorithm.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>