Merging ros_control_gazebo and gazebo_ros_control

gazebo_ros_control assumes that all of the information needed to connect gazebo and ros_control can be found in the URDF. ros_control_gazebo instead requires that the user write a plugin for doing this. This new merged version uses @davetcoleman's version by default, as gazebo_ros_control_plugin::DefaultRobotSim, but alternatively allows a user to implement a custom plugin which gives him or her access to the full gazebo API for things not modeled in URDF.

Other changes:
 * ros_control_gazebo --> gazebo_ros_control_sim_interface
 * ros_control_gazebo_plugin --> gazebo_ros_control_plugin
 * ros_control_gazebo_tests --> gazebo_ros_control_examples
 * Added metapackage "gazebo_ros_control"

I'd also like to get feedback from @adolfo-rt and @yamokosk