Adding an implementation of `ssn` provider for the it_IT locale.

This implementation generates fake syntactically correct italian fiscal codes.
The checksum character is calculated accordingly to
http://en.wikipedia.org/wiki/Italian_fiscal_code_card.
`encode('ascii')` is making this fail on Python 3 and I don't see why it's necessary. `code = cls.bothify(cls.fiscal_code_format).upper()` should just work.