Ruby Challenges 8, 18, 21, 23 and 82

 new file:   Ruby/challenge8.rb - Reverse words
 new file:   Ruby/challenge18.rb - Multiples of a Number
 new file:   Ruby/challenge21.rb - Sum of Digits
 new file:   Ruby/challenge23.rb - Multiplication Tables
 new file:   Ruby/challenge82.rb - Armstrong Numbers