Refactor Model: simplify list/1 and rename opts to _opts

I noticed while watching the elixir sips episode where it was introduced that `Model.list/1` was just doing a map, and thought I'd suggest the simplification of just using `Enum.map/2`.

While I was there I noticed the compiler was warning about `opts` not being used, so I prefixed it with an underscore for the time being.