Efficiency improvements and customizable occupancy 2D map projection.

+ Efficiency improvements:
|- avoid transforming clouds already in map frame (added additional
parameter to specify sensor frame_id for calculating sensor pose)
|- create occupancy cloud directly as ros message instead of pcl cloud
(avoids conversion)

+ Projection of nav_msgs::OccupancyGrid with customizable height and
initialization as free or unknown.

+ Increased tf listener buffer to 30 sec.
I didn't consider that case, because i assumed no one would want to insert scans in the m_worldFrameId without providing the sensor frame id.
In fact, i introduced this extra parameter, because if the cloud is in the m_worldFrameId (map), the raytracing algorithms won't work as expected because they would be casting rays from the wrong origin.
Nevertheless, i see your point, and the code should be robust to handle incorrect parameterization.
Moreover, it may be useful to provide point clouds in other frames other than map and sensor, so this parameter would be useful for fixing the raytracing origin.