Add shebang to list_lab. Submit dict/set lab, mailroom, safe_input

All of the assignments being submitted here are complete. Are we supposed to make a pull request with a different branch anyway?
You should limit the amount of whitespace to a line or two that delimit logical groupings of code. Many production python shops would consider this much whitespace to be a bug.

See http://legacy.python.org/dev/peps/pep-0008/#blank-lines