Overrides formfield method on Brazilian CPF, CNPJ and PostalCode model field.

[lucasgueiros]

**Overrides formfield method on Brazilian CPF, CNPJ and PostalCode model field. #489 **

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All Changes

• Add an entry to the docs/changelog.rst describing the change.

• Add an entry for your name in the docs/authors.rst file if it's not
  already there.

New Fields Only

• Prefix the country code to all fields.

• Field names should be easily understood by developers from the target
  localflavor country. This means that English translations are usually
  not the best name unless it's for something standard like postal code,
  tax / VAT ID etc.

• Prefer 'PostalCodeField' for postal codes as it's
  international English; ZipCode is a term specific to the United
  States postal system.

• Add meaningful tests. 100% test coverage is not required but all
  validation edge cases should be covered.

• Add .. versionadded:: <next-version> comment markers to new
  localflavors.

• Add documentation for all fields.