Added MarshalJSON to Vin and changed Vout.ScriptPubKey

Added MarshalJSON method to Vin which properly omits the Vout field
from coinbase Vin's by using anonymous structs with the proper
subsets of fields outputted by bitcoind

Changed Vout.ScriptPubKey to add omitempty for ReqSigs and Addresses:
both fields are not shown when there is an error identifying to
scriptType or parsing addresses in bitcoind