I added a few more problem types to permutation_1.  This is my first attempt to contribute to Khan Exercises, so feel free to be brutally honest and complete with any criticism or advice.

Added 3 more problem types: 
  stacking colored blocks, 
  rearranging digits in a number, and 
  travelling salesman sequencing.  
Equalized data-weights so all 5 problem types are 
selected with equal probability.