Added all SEPA countries (IBANs from European Union countries) and minimal test coverage for countries

lib/iban_ex/country/at.ex

@@ -1,6 +1,13 @@
 defmodule IbanEx.Country.AT do
   @moduledoc """
   Austria IBAN parsing rules
+
+  ## Examples
+
+    iex> %IbanEx.Iban{country_code: "AT", check_digits: "61", bank_code: "19043", branch_code: nil, national_check: nil, account_number: "00234573201"}
+    iex> |> IbanEx.Country.UA.to_string()
+    "AT 61 19043 00234573201"
+
   """
 
   @size 20