created current user/logged in helper things

Well, new helpers, but they're functions in the application helper,
which gives us access to them all over the place.

Basically, there's one to check if someone is currently logged in, and
there's one to get the logged in users's object, so you don't have to
write:

  if logged_in?
    User.find_by_id(session[:user_id]).something
  end

You can now call:

  if logged_in?
    current_user().something
  end

Nicer, right? It returns false if there isn't anyone logged in right
now.

I also put some demo useage in the application layout, showing the login
form is there isn't a logged in user, but showing a hello message if
there is. Look there for an example if you need one.