Conditional statements 2 exercise

Added exercise as specified in https://trello.com/card/conditional-statements-2/4d87e664967a0775082939ab/423
Note that I did not implement problem type 3 - see my comment in the Trello card because the statement should be corrected ("If a number is divisible by 5, then its last digit is a 5").
http://sandcastle.khanacademy.org/media/castles/garypoison:071b79a333ddfa6139daa7b49da605a179343304/exercises/conditional_statements_2.html