Implement some kind of off-chain computation

[lajarre]

Existing solution: https://github.com/pipermerriam/ethereum-computation-market

Or a very simple one, for a given graph calculation:

• an oracle asks for a solution, someone posts it with a deposit that should be big enough to account for the cost of the graph calculation if it is done on the blockchain
• before the end of a 7-day period, anyone has the right to challenge the computation, betting the double amount than the one of the deposit
• if nobody challenges: the first person to answer to the oracle gets a pre-defined gain
• if someone challenges and he's right: the first person loses his deposit, the second person gets all the money that is left after the ethereum computation has been done
• if someone challenges and he's wrong: everybody loses?