Move JTC action server and trajectory sampling into separate class

[nbbrooks]

Add new file trajectory_execution_impl.hpp, which is responsible for:

• setting up and managing a control_msgs::action::FollowJointTrajectory action server
• checking/managing the current Trajectory
• in general, any sort of manipulation/validation of JointTrajectory messages

The idea is to pull out this common functionality from JointTrajectoryController so that it can be easily repurposed in other controllers want to handle JointTrajectory messages.

For future thought, we could potentially add new functions or comments to ControllerInterface to make it more clear the typical steps done in the update() function.