add dummy joint trajectory server and update demo_sim.launch

When I found demo_sim.launch, I thought this is the launch file for those who do not have real baxter nor baxter gazebo simulation, but it seems not.
One easiest solution is to set `allow_trajectory_execution` to false (http://moveit.ros.org/wiki/MoveGroup_Node) to disable `Execute` button. but I do no like to see the baxter in the rviz is still in initial position.
If MoveIt! provides something like loopback trajectory execution, that just to update robot posture from trajectory execution, I'd like to use that.

This PR contains joint_trajectory_controller that takes FollowJointTrajectory and publishes joint states, so if you launch demo_sim.launch, you can use MoveIt without realrobot/gazebo.
+1 for code credit. If any portions were straight copies of the original library, they should be marked as such