gazebo_ros_api_plugin.h is no longer exposed in the include folder

@jbohren said in https://github.com/osrf/gazebo_ros_pkgs/pull/35
> Why does gazebo_ros_api_plugin.h even need to be exposed? Should anything other than gazebo_ros_api_plugin.cpp be including this header? If not, I think they should both be in the src directory, and the include directory in the gazebo_ros package, can be blown away.