Add sign reward for each transaction to encourage wallet/client to support QuarkChain

[qzhodl]

There is block reward for miners to package transactions, but there isn't reward for wallet/client to sign transactions. Suggest to add a "coinbase" field in the transaction structure to record the address of signer which should be the wallet/client.

When the miner package the transaction in a block, they need add rewards for all the signers of the transactions in the block. The reward should be very small compared to the miner reward and can be decreased to zero when there are a lot wallets that support QuarkChain.