Rough ROS 2 port

[sloretz]

This is a rough ROS 2 port that is enough to parse a collada model of a PR2.

The code here is littered with TODO statements - mostly for how errors are logged. I haven't benchmarked it, but this plugin seems to be very slow. It takes a few seconds to load a collada file of a PR2.

The target branch should be changed to a to-be-created ros2 branch.

@scpeters @iantheengineer FYI

requires ros2/urdf#13