String to bytes

Looking through the source I noticed that the data being read from redis starts as `[]byte` but is always parsed into `string`. There's even a method `Bytes()` which then parses it *back* into `[]byte` for those who want it. If a user was to want a BulkReply in the form of `[][]byte` they would have to loop through the whole dataset again. This seemed like a waste to me, since the data starts off as `[]byte` why not just keep it that way.

My change is that Reply now holds it's data as a `[]byte` instead of a `string`. All functions stay the same and their behavior hasn't changed, and I added a new function, and test cases for it, called ListBytes, which returns the `[][]byte` of a BulkReply. All tests pass on my machine.