Shutdown segfault fix

Lots of changes in this pull request, it would be great if someone could test it before merging it

    CHANGES
    fixed gazebo unload error - no more segfaults
    switched to async ROS spinner
    Removed the physics reconfigure node handle
    reduced required while loops
    renamed rosnode_ to nh_ 
    renamed member variable world -> world_
    wrench_body_jobs -> wrench_body_jobs_
    removed all "this->" references since not needed
    formatted functions