Python: added ROS back-end

Backends are automatically detected at module initialization.
There will be a warning if more than one is available and
you didn't specify the backend config key, e.g. using the env:
export IPAACA_BACKEND=ros
or
export IPAACA_BACKEND=mqtt

TODO: ROS host / port configuration etc (but rospy probably
honors their normal env vars internally; not tested)