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, in this instance `wobj_base_` and `tool_base_` are frame id's (strings).  I think they will be needed in case where the ik solver base/tip links do not match those assumed by trajectory points.

For example, our use case has a calibrated tool frame and world frame, defined in the URDF.  This means the trajectory point will have be relative to these frames.  This will result in a cartesian point with wobj and tool initialized to identity.