add route to delete messages

Just merge this pull request, then from your command line, type `git push heroku master`, then go to the url:
http://whispering-bastion-3471.herokuapp.com/messages/9/delete

This will remove the bobby droptables entry and protect you from future html injection attacks. See this article for more info on Sinatra and sanitizing inputs: http://www.sinatrarb.com/faq.html#escape_html

Afterwards, you should remove the controller action for deleting entries (or at least restrict it so no one else can use it)