Adds Simulated Electric Parallel Gripper

[IanTheEngineer]

This PR adds the Electric Parallel Gripper (EPG) support to Sawyer Gazebo. The changes consist of

• Adds launchfile args necessary for electric_gripper. Invoke with $ roslaunch sawyer_gazebo sawyer_world.launch electric_gripper:=true
• Adds a (moderately tuned) ros_controls PID controller yaml for the electric gripper
• Changes the way ros_controls controllers are loaded. Some are now spawned to start them by default. These include: joint_state_controller, head_position_controller, right_joint_gravity_controller, right_joint_position_controller and the electric_gripper_controller when it is specified
• Finally, adds a class ros_controllers class to accept IO Framework gripper commands (the same way it is done with the real robot), and respond with IO Framework style responses

[IanTheEngineer]

Ok, all the required updates are made. I just need to squash it into one-ish commits and give it a good message. To be continued!