Update contact manager for Gazebo 2.0

I've updated the gazebo create plugin for Gazebo2. It's late and I don't have time to test it, but it at least seems sound and it compiles.

This should resolve #7 

It may also be worth consolidating the hydro, hydro-devel and indigo branches; it looks like they've diverged, and there are changes in the hydro branch that haven't made it to the indigo branch yet...