The string needs to be unmarshaled rather than returned directly otherwise
it will have an extra set of quotes plus it's safter to properly unmarshal
it.
Pointed out by @jrick.
The submitblock RPC returns a string containing an error if it failed for
any reason. This comment detects a non-null return from submitblock and
converts that string to an error which is returned.
There are several RPCs which accept a pointer to a hash, transaction,
block, etc. Previously not all RPCs handled being passed a nil pointer
consistently.
Closes#4.
