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.

> Could you add a link to more info about SDF estop feature?

E-stop is a feature of ros_control, not SDF. As far as I know, it is documented only in @adolfo-rt's 2014 ROSCon presentation. Go to http://vimeo.com/107507546. Adolfo begins talking about ros_control's E-stop feature at 27:14. We have been discussing SDF because that's where the parameter that defines the E-stop control topic is specified.