cleaned up urdf descriptions and fixed horizontal_fov tag in generic_camera.urdf.xacro
hector_sensors_description: use gazebo_ros_camera plugin instead of gazebo_ros_depth_camera for the generic camera

* This was a relict from gazebo for fuerte.
hector_sensors_description: added some more missing xacro namespace prefixes
bumped version to 0.2.3
hector_sensors_description: fixed scale of asus_camera and kinect_camera meshes