Add support for PX4 and running in SITL

[tfoote]

These changes let you run the demo in gazebo with the PX4 SITL stack.

Here's a screen capture of the resultant demo: https://www.youtube.com/watch?v=wnfG5ulkkek

I tried to make sure all the changes are backwards compatible for the current demo.

The other change is that it makes the simulation version use a generic ROS joystick instead of the Artoo class. In the future it might make sense to create a ROS node that publishes standard Joy message publisher from the Artoo. This would decrease the difference between operating on a live system and a simulated one as well as transitioning between vehicles with potentially different controllers.

This is leveraging some of the infrastructure from an ongoing project available at: https://github.com/osrf/uctf In the future I will work to document how to reproduce this demo in full once the development on uctf slows down.

[imcnanie]

Fantastic! Great job getting it working, I'll review and pull it in. Let me know if you have a project you want to build with this, I (and the others that worked on this) would love to help.

[cynthiamoon]

@tfoote i want to run multiple autopilots in one SITL gazebo.i know different physical objects in Gazebo must have different names.i don't know how to add another autopilots on one SITL gazebo.
i use Px4 firmware.

[imcnanie]

@cynthiamoon Check out this doc: https://docs.google.com/document/d/1heLTpFEyNC_52BnZnz78lZxPeW15iZX98VUEXM4GIBM/edit# Specifically the end. Let me know if you have questions.