Don't offset the fifth joint of the arm.

It is not a real redundancy, instead it only looks like one when
symmetric grippers/tools are attached to the arm.
Fix bug in the inverse kinematics for side grasps.

The IK solver is now able to correctly solve for poses which are on the
left/right (positive/negative y axis) of the arm. This seems to be an
error in the Craig book from where this solution originates.
Update comments.

Explain the distance between the fourth joint and the end-effector frame
(i.e. the fifth joint).
Add simple MoveIt! kinematics plugin.

The current version is basically empty, but can already be loaded via
the pluginlib loader.
Initialization extracts required data.

When the MoveIt! plugin is initialized, it extracts the joint names,
link names and joint limits from the URDF model which it finds on the
parameter server.
Add auto-generated MoveIt! configuration package.
Include the inverse kinematics.

The plugin now uses the inverse kinematics from the
youbot_arm_kinematics package and offers its capabilities to MoveIt!.
Number of joints accessible in complete class.
Validate the seed before running the kinematics.