rosnode: Use policy.init for latching in ROS topics

Suggested by Miguel Prada.

Signed-off-by: Peter Soetens <peter@thesourceworks.com>
cmake: fix linking with roslib.

Before was implicitly through RTT.

Signed-off-by: Peter Soetens <peter@thesourceworks.com>
Merge branch 'master' of http://git.mech.kuleuven.be/robotics/rtt_ros_integration