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 made this `mutable` to get around the const methods issues (`robot_state_` non-const methods are required to calculate things like IK.  I'm not sure of all the implications of this.  One potential issue (maybe only in C++11) is that this class is not thread-safe (see [here](http://channel9.msdn.com/posts/C-and-Beyond-2012-Herb-Sutter-You-dont-know-blank-and-blank), about 25:00 minutes in).  This should be captured in an issue or if anybody has any suggestions, I'm open.