Fix deserialize bytes in Python3

See #9 and https://github.com/laas/morse/issues/218

`data` in Image and PointCloud2 should be kept as bytes, not decoded as utf-8.

Tested with:
```
./scripts/genmsg_py.py -p sensor_msgs -Istd_msgs:`rospack find std_msgs`/msg -Isensor_msgs:`rospack find sensor_msgs`/msg -o gen `rospack find sensor_msgs`/msg/Image.msg

diff -ur1 /opt/ros/fuerte/lib/python2.7/dist-packages/sensor_msgs/msg/_Image.py gen/_Image.py
```