Added gen_ipv4() and gen_ipv6() methods

Added two new generators to generate valid IPv4 or IPv6 addresses. These values can be used with [IPAddressField](https://docs.djangoproject.com/en/dev/ref/models/fields/#ipaddressfield "Model field reference | Django documentation | Django") and [GenericIPAddressField](https://docs.djangoproject.com/en/dev/ref/models/fields/#genericipaddressfield "Model field reference | Django documentation | Django") fields