ABM.DatGUI folders

You can make dat.gui folders by nesting objects. E.g.:

            {
              "Moose stuff": {
                "Create a moose": {
                  type: "button"
                },
                "Moose color": {
                  type: "choice",
                  vals: ["brown", "dark brown"]
                }
              },
              "Goat stuff": {
                "Look for a goat": {
                  type: "button"
                },
                "Goat size": {
                  type: "slider",
                  min: 1,
                  max: 9000
                }
              }
            }

Also, ABM.DatGUI now emits `'change'` events whenever its model changes.