Rather than using bytes.NewBuffer, which is a read/write entity
(io.ReadWriter), use bytes.NewReader which is only a read entitiy
(io.Reader). Benchmarking shows it's slightly faster and it's also
technically more accurate since it ensures the data is read-only.
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.
