Add new RpcRawCommand to get []byte represtation of reply instead of
btcjson.Result.
Export ReadResultCommand to allow caller of RpcRawCommand to deal with
the reply in the same way RpcCommand does.
Adjust test code to for the above changes.
Add return types for serveral additional commands.
Fix required vs. optional arguments for createrawtransaction.
Update tests and test coverage for all changes.
Following the example of bitcoind and the bitcoin wiki, provide a
function to convert floats (which are returned by the json-rpc server)
to int64 (which is the normal representation for most values involving
bitcoins).
