Handle "before YYYY" entries

[BillWeissABK]

Pretty trivial change, but it seems to work :)
Fixes #228

Before:

>>> import whois
>>> whois.whois("ns.tomis.ro")
{'domain_name': 'tomis.ro', 'domain_status': 'OK', 'registrar': 'ICI - Registrar', 'referral_url': 'http://www.rotld.ro', 'creation_date': 'Before 2001', 'expiration_date': datetime.datetime(2028, 5, 29, 0, 0), 'name_servers': ['ns1.lucaservices.ro', 'ns2.lucaservices.ro'], 'status': 'OK', 'dnssec': 'Inactive'}

After:

>>> import whois
>>> whois.whois("ns.tomis.ro")
{'domain_name': 'tomis.ro', 'domain_status': 'OK', 'registrar': 'ICI - Registrar', 'referral_url': 'http://www.rotld.ro', 'creation_date': datetime.datetime(2001, 1, 1, 0, 0), 'expiration_date': datetime.datetime(2028, 5, 29, 0, 0), 'name_servers': ['ns1.lucaservices.ro', 'ns2.lucaservices.ro'], 'status': 'OK', 'dnssec': 'Inactive'}

[richardpenman]

Thanks for all the fixes Bill!