Add validation of ID card number format, placeholder for form.

Fixes #37
I usually try to stick to one assert per spec:

```ruby
it 'must be in the correct format' do
  user = User.new(:identity_card_number = '123-456789-1234A')
  expect(user).to have(0).errors_on(:identity_card_number)
end

it 'does not allow an invalid format' do
  user = User.new(:identity_card_number => 'ABCD1234xyz')
  expect(user).to have(1).error_on(:identity_card_number)
end
```