Users can create lists

[Trello card - Users can create lists](https://trello.com/c/1g70Hzpx/5-as-a-user-i-ll-be-able-to-add-drinks-to-my-list-and-figure-out-all-the-ingredients-i-ll-need-to-make-the-drinks)

Also quantity of each ingredient needed in a recipe is now stored in base units of fluid-ounces.
Aha, how about `amount.convert_to(unit_of_measure)`?