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.
You're missing a dependency on `cmake_modules`: `FindEigen.cmake` is not part of CMake itself, but is distributed as part of `cmake_modules`, a ROS package.