[Iris] add example for Iris based upon the edifice-demo depot world

[srmainwaring]

This PR adds another example for the Iris quadcopter. The world is a modified instance of the Edifice demo world.

The main additions to the model are:

• Include the iris_with_standoffs model in the OSRF Edifice demo
• Add the ignition::gazebo::system::Imu plugin.

The world may be launched with:

$ ign gazebo iris_arducopter_edifice_demo.world

and SITL with:

$ sim_vehicle.py -v ArduCopter -f gazebo-iris --model=JSON --console --map