Solutions for Lists and Recursion (ch07)

 * Exercise 0 - http://forums.pragprog.com/forums/322/topics/11929
   `sum` function` in `my_list.exs`
 * Exercise 1 - http://forums.pragprog.com/forums/322/topics/11930
   `mapsum` function` in `my_list.exs`
 * Exercise 2 - http://forums.pragprog.com/forums/322/topics/11931
   `max` function` in `my_list.exs` - NOTE use Kernel.max next time
 * Exercise 3 - http://forums.pragprog.com/forums/322/topics/11932
   `caesar` function in `my_list.exs`
 * Exercise 4 - http://forums.pragprog.com/forums/322/topics/11934
   ` span` function in `my_list.exs`