Brokers on the whitelist can be removed

[patriacaelum]

At the moment, the WhitelistMechanism is designed to be a one way system. Once brokers are whitelisted, they stay there permanently. When a vote for a broker is changed, this may affect a broker's whitelist status.

However, this means that a broker that is already committed to a proposal may be removed from the whitelist, but still remain a broker on the proposal. Whether or not the broker should be immediately removed from the proposal is an open question.

• Keep past voting information in the WhitelistMechanism
• Add a remove_from_whitelist method
• When a voter votes, consider what happens if the broker is already in the whitelist. If the minimum condition is not longer met, then the broker is removed from the whitelist and added back to the waitlist
• Add removing unit tests