Add DepthImage observation source

This allows to generate the clearing `xyz` points using the `PinholeCameraModel` of the depth images, for those pixels/points where there's no "echo"/"return" (invalid pixels).

See https://github.com/ros-perception/image_pipeline/pull/104

This PR also needs https://github.com/ros-perception/image_pipeline/pull/108 to be merged before, since it tries to avoid copying code from `depth_image_proc`. This other PR simply exports some headers and extends the `convert` function to allow for setting `range_max` for invalid pixels.
You're right, I shouldn't have add this change here.
I think it's strange that other params (also in the specific costmap namespace) are retrieved only with a simple `source_node.param`, isn't it?. I think these two other params can be retrieved the same way.

I'm going to remove the commit that did this... but if you want I can send it on a separate PR.