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.
@dpsolomon, I must have missed your comment before I merged.  I'll open an issue to discuss this further.

@jrgnicho, I'll open an issue for your comment as well....although it's more straight forward.