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 would rather see wobj_base_ and tool_base_ always be just outside the 1st and last link (respectively). They can be initialized to Identity by default.