Add sg (Singapore) flavor

- Phone number
- Postal code
- National IC/Foreign ID

P.S. Sorry I am a newb. This is my first pull request. Thank you for your consideration.


Actually, RegexValidator doesn't have min_length and max_length: you are overriding the message and code arguments now: https://docs.djangoproject.com/en/dev/_modules/django/core/validators/#RegexValidator