RFR - Add factory girl

Instead of doing this, create a new migration to add the username column.

Otherwise, when your collaborators pull this down, they won't be able to update their databases unless they run `rake db:rollback first`.  That's not good.  So you want to run `rake db:rollback` on your own machine, then generate a new migration to `add_column :users, :username, :string, null: false`.  You'll also want to add a uniqueness constraint to that column in your migration.