Moved robot_state library code into robot_model library.

This is a (likely) necessary precondition to writing the new inverse-kinematics API I would like to write.

Without this change my IK code would create a circular dependency between the libraries, and I think that is at least a bad idea and maybe doesn't work at all.

Currently moveit_core consists of around 20 tiny separate libraries, each of which depend on several others in a complicated dependency graph.  External users of these libraries seem to just use the catkin build machinery to say "I need moveit_core" which gets them the whole set of libraries.  So no one outside of moveit_core actually cares about library names.

I think it may be a good idea to combine many more and simplify the build structure, but for now I'm just doing the one I need.