Exapand proof submission request for independent or pooled mining

[hxw]

Add additional request fucntion to proof/submission.go. This currently only supports returning a nonce. Add the txZero field to the request and add a hash field to the response.
The submission must verify that txZero is a valid foundation record before accepting the submission and only return the hasg if this record is valid.

This allows for two possibilities

1. recorderd can sign its own txZero and thus provide its own payment addresses

2. a mining pool program can be created to use the returned hashes to apportion rewards to individual miners