new file:   prob_9.rb
problem: special pythagorean triplet, http://projecteuler.net/problem=9
new file:   prob_10.rb
problem: summation of primes, http://projecteuler.net/problem=10
new file:   prob_11.dat
new file:   prob_11.rb
problem: largest product in a grid, http://projecteuler.net/problem=11