Support sealed bid auctions

[dob]

In a sealed bid auction, bids are collected but are not unsealed until a specified date. On a blockchain one way to do this might be to submit a hash of a bid + nonce during the bidding phase. And then during an unseal phase they reveal their bid + nonce, and their contract confirms its accurate and not changed via the hash. The highest revealed bid wins.

Weaknesses include that the bid amount could theoretically be revealed via data in a spam transaction, and that if a party forgets to reveal their bid then they may be out of luck. An integration with the ethereum alarm clock could remedy this, but we'd have to think about how to do that without revealing the bid.