Fix roslib import for message module, remove roslib.load_manifest

The following code snippet fails with the current code base:
```python
#!/usr/bin/python

from sensor_msgs.msg import PointCloud2
from sensor_msgs import point_cloud2

cloud = PointCloud2()
[[p[0], p[1], p[2], 1] for p in point_cloud2.read_points(cloud)]
```

points = [[p[0], p[1], p[2], 1] for p in point_cloud2.read_points(point_cloud)]
  File "/opt/ros/hydro/lib/python2.7/dist-packages/sensor_msgs/point_cloud2.py", line 74, in read_points
    assert isinstance(cloud, roslib.message.Message) and cloud._type == 'sensor_msgs/PointCloud2', 'cloud is not a sensor_msgs.msg.PointCloud2'
AttributeError: 'module' object has no attribute 'message'