Header files of packages gazebo_ros and gazebo_plugins are installed to the wrong location

I recreated this pull request from #32 as I did not create a branch and pushed more commits in my hydro-devel branch in the meantime. I am sorry.

The packages gazebo_ros and gazebo_plugins install their header files to `${CATKIN_PACKAGE_INCLUDE_DESTINATION}` as in the catkin example at [ros.org](http://ros.org/wiki/catkin/CMakeLists.txt#Optional_Step:_Specifying_Installable_Directories), but as the include directory contains another subdirectory named gazebo or gazebo_plugins headers are installed to

* include/gazebo_ros/gazebo/gazebo_ros_api_plugin.h
* include/gazebo_plugins/gazebo_plugins/gazebo_ros_camera_utils.h
* ...

This patch changes the installation target directory to `${CATKIN_GLOBAL_INCLUDE_DESTINATION}`, so that header files end up in

* include/gazebo/gazebo_ros_api_plugin.h
* include/gazebo_plugins/gazebo_ros_camera_utils.h
* ...

We are using the `GazeboRosCameraUtils` class in our [hector_gazebo](http://www.ros.org/wiki/hector_gazebo) stack and include the header using the `#include <gazebo_plugins/gazebo_ros_camera_utils.h>` which would fail without this patch.

See also my question regarding installation of header files in catkin packages on [ROS answers](http://answers.ros.org/question/65716/which-is-the-correct-way-to-install-header-files-in-catkin-packages/).