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'll add an issue for this.  My approach is bad for several reasons.  I'm just trying to get something that will quickly create multiple solutions to populate the graph.