Pull Request for new servo driver code

This branch makes several changes to the ur_driver package:
* Changes the [ur5.urdf.xacro](https://github.com/jhu-lcsr-forks/universal_robot/blob/390cac22b45a2a2bcb66ae3547aad14026bc3861/ur_description/urdf/ur5.urdf.xacro) file to properly represent the base of the robot relative to its first joint.
* Adds a new servo_driver.py file which is a clean implementation of driver.py.  It does not include the trajectory actions in driver.py.  Those will be added in the future. There is also a launch file, several services and program (.prog) files to support this interface.

  To run the robot with this new interface:
  ```bash
  roslaunch ur_driver test_servo_driver.launch robot_ip:=192.168.1.155
  ```
* Adds two interfaces for interacting with the robot
  * A generic robot status panel
  ```bash
  rosrun ur_driver ur_status_interface.py
  ```
  * A waypoint following panel
  ```bash
  rosrun ur_driver marker_teleop_interface.py
  ```
* Changes to CMakeLists.txt and package.xml to accommodate new services for servo_driver.py