This fixes #50, and makes ros/ros_comm#781 easier to fix

`struct.pack('5s', b'abcde')` works just fine for me on 3.5.0

And so `chr` is also not safe. `b'\0' works fine in both versions.

This way things like `np.float32` and `np.int64` will be accepted as well.

And fix them to return integers not floats for floor division, to match `datetime.timedelta`