[Indigo] Initial version GazeboRosControlPlugin with Multi-HWInterface-Support

This feature has originally been proposed to [gazebo_ros_pkgs](https://github.com/ros-simulation/gazebo_ros_pkgs).
I'm posting the relevant PullRequests for the record:
[gazebo_ros_pkgs#256](https://github.com/ros-simulation/gazebo_ros_pkgs/pull/256)
[gazebo_ros_pkgs#266](https://github.com/ros-simulation/gazebo_ros_pkgs/pull/266)
[gazebo_ros_pkgs#267](https://github.com/ros-simulation/gazebo_ros_pkgs/pull/267)

This feature still requires [ros_control#184](https://github.com/ros-controls/ros_control/pull/184) to be merged!

Additional features will follow...