Changed subscriber to pcl::pointcloud

The way the subscriber is initiated is changed according to pcl_ros library. This saves two casts from pointcloud msg to pcl. (And appears to reduce the execution time inside the callback)

My only concern is: I don't understand how the subscriber obtains the pcl cloud from the ROS messaging system. Subscribing to a pcl pointcloud might therefore have induced delay outside the callback, but I was unable to test this.

(Offtopic: I have also tried out chilimarkers in stead of artoolkit, this seems to further decrease delay and appears slightly more robust, fork is on github)