Add Monoid instance for Bytes

A monoid instance for `Bytes` would be handy for the `linesIn` process I've written for my Subprocess adventure: https://github.com/fthomas/scalaz-stream/blob/c8d54905d6fd33986874c7aff605257819574d87/src/main/scala/scalaz/stream/Subprocess.scala#L164
So here it is ...