Added Ruby challenges 10, 31, 67, 91 and 100

 new file:   Ruby/challenge10.rb - Mth to last element
 new file:   Ruby/challenge31.rb - Rightmost Char
 new file:   Ruby/challenge67.rb - Hex to Decimal
 new file:   Ruby/challenge91.rb - Simple Sorting
 new file:   Ruby/challenge100.rb - Even Numbers