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 robot's simulated hardware indicates that the emergency stop has been deactivated after being activated. DefaultRobotHWSim now subscribes to the Boolean topic specified by the eStopTopic SDF element. 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.

We could also parse the e-stop topic name from the ROS param server and not break the API, but that would fragment the configuration of a `RobotHWSim` plugin to multiple places (SDF, param server), which I think is undesirable.