Added emergency stop support

I have added emergency stop support to the gazebo_ros_control and DefaultRobotHWSim plugins. gazebo_ros_control now resets the controllers when the emergency stop has been deactivated after being activated. The eStopTopic SDF element defines the E-stop topic. Messages received from the topic control the E-stop. When the E-stop is activated, the following occurs:

* If a joint's position is being controlled, DefaultRobotHWSim maintains the position the joint had when the E-stop was activated.

* If a joint's velocity is being controlled, the velocity is set to zero.

* If a joint's effort is being controlled, the effort is set to zero.