Add proof_key in response for bid_tools get.

[hgedia]

$ python3 scripts/bid_tools.py get --key=84304538
Limit bid:       {
    "_key": "84304538",
    "cost": 7,
    "sender": "robertogomes",
    "statement_key": "79169223",
    "status": "completed"
}

Currently proof-key is not returned in the response and we should add this to prevent users from using the -v flag.