Implement escrow/dispute management

[bitgamma]

At the moment all payment transactions are final and the funds are directly transfer from buyer to merchant. We need a mechanism to revert malicious transactions. What we already mitigate is

1. Caching of transactions. When a transaction is created it expires after x (currently 10) blocks are elapsed, so it must be sent immediately on creation.
2. Multiple transactions. The contract only accepts payment requests at least x (currently 5) blocks apart, so in a single tap you can only create 1 valid transaction.
3. Maximum tx amount. This sets the limit on how much can be transferred in one payment.

What we want to mitigate is the merchant showing a different amount on the POS screen from what is actually being requested and also unauthorized taps. An escrow and a reputation system could help identify and punish malicious actors.