[day-02] add user schema and migration

Add elixir struct to represent an user and its corresponding
representation on the database.

lib/wabanex/user.ex

@@ -0,0 +1,13 @@
+defmodule Wabanex.User do
+  use Ecto.Schema
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+
+  schema "users" do
+    field :name, :string
+    field :email, :string
+    field :password, :string
+
+    timestamps()
+  end
+end