Add Python solution to question 9-1 (staircase)
Add Python solution to question 9-2 (robot)
Add Python solution to question 9-3 (magic index)
Add Python solution to follow-up of question 9-3 (magic index)
Add Python solution to question 9-4 (subsets)
Add Python solution to question 9-5 (permutations)
Add Python solution to question 9-6 (parentheses)
Add Python solution to question 9-7 (paint fill)
Add Python solution to question 9-8 (change-making)
Add Python solution to question 9-9 (n-queens problem)
Add Python solution to question 9-10 (box stacking)
Add Python solution to question 9-11 (boolean expression)
Add Python solutions to Chapter 9 (Recursion and Dynamic Programming)

This merges branch 'python-chapter-9'