Added Moveit adapter package

This implements the Descartes `RobotModel` interface for MoveIt.  The PR includes some parts of PR #13, but not all.  Method implementations for `TrajectoryPt` are not included.  However, these methods will simply be wrappers around RobotModel calls when required.
I added this dependency only to find out that I didn't have `cmake_modules` installed.  Everything worked before adding the dependency, so I'm not sure it is really needed.  I think the dependency on `moveit_core` pulls in `Eigen`, though I'm not sure.