rtabmap: added "flip_scan" parameter to flip scan values if the laser rangefinder is upside down (fixed #65)

Added support for input scan cloud with normals and add parameter "scan_cloud_normal_k" for convenience to compute normals from scan cloud (if they don't have normals)

Obstacles detection: added output "proj_obstacles" for a cloud of projected obstacles on xy plane. Fixed not published /scan_map when scan_voxel_size=0.0.

cmake: Re-added optional_dependencies in catkin_package() to avoid linker errors on packages depending on rtabmap_ros

Nodelets point_cloud_xyz[rgb]: fixed NaN error when no prior filtering is done before radius filtering

removed optional libs from CATKIN_DEPENDS to avoid catkin error (catkin_package() DEPENDS on the catkin package 'costmap_2d' which must therefore be listed as a run dependency in the package.xml)

CoreWrapper: using twist covariance instead of pose covariance (which could grow out of bounds depending of the odometry used)