RViz interface: virtual occlusions & pointing buttons

@mateiciocarlie @jon-weisz 

Adds RViz part of #30 and a virtual "invisible robot" that occludes interactive markers etc. in the first-person view. Also, there are now two pr2_marker_control nodes, the old one for the web interface and a new one that only overlays transparent controls over the robot model in rviz.

There will be three new buttons which simply publish a PointStamped when you click on something, their backend functionality still needs to be connected.

Requires some unmerged API changes to RViz core, so use together with https://github.com/dgossow/rviz/tree/all_merged