Banned/cleared domains API

[sdgathman]

When used as part of a larger reputation system, pygossip needs to be told about domains that have been whitelisted or banned by means other than being marked as spam. E.g. domains that send spam to a honeypot are instantly banned, and pydspam will soon have a feature to click a button to ban an authenticated domain (that is clearly fraudulent, for example).

The new API features should also support whitelisting, e.g. bmsmilter can auto-whitelist recipients of outgoing mail, and pydspam should have a whitelist button.

So a whitelisted domain should report best possible reputation (+100), and blacklisting should result in worst possible (-100).