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.
This parameter is needed when the pointclouds arrive already in the map frame (for example in a 3D pointcloud assembler from 2D laser scans). 
It is necessary to find the sensor pose to integrate the pointcloud in the octree.