Define encrypted ActiveRecord methods inside a module, so we can use super

(pattern extracted from: http://thepugautomatic.com/2013/07/dsom/ )

You can then, for example, migrate to encrypted records one by one, when they change:

```
class EncryptBankAccount < ActiveRecord::Migration
  def change
    rename_column :users, :bank_account_number, :clear_bank_account_number
    add_column :users, :encrypted_bank_account_number, :text
  end
end

class User < ActiveRecord::Base
  attr_encrypted :bank_account_number
  def bank_account_number
     super || clear_bank_account_number
  end
end
```

And later, drop the `clear_bank_account_numner` and delete the `bank_account_number` method.