automatic verification of subdomains of already verified domains - GH54

1. If a domain is added by a user, and that user has a validated superdomain of the new domain, automatically mark the new domain as validated.
2. If a new domain is added, check that it is not a public suffix.

Note: new dependency added ([PublicSuffix](http://pypi.python.org/pypi/publicsuffix/)), run `python setup.py develop` to pull it.