Fixing gazebo_bridge gzmath include, minor bugfix, and small increment

Fix 1:
#include <math/gzmath.hh>
should be
#include <gazebo/math/gzmath.hh>

This will enable compilation on a brand-new environment.

Fix 2:
I have fixed the use of ros_odom_frame, which was being ignored. It was using hardcoded "/odom"...

Increment 1:
I've added a ros_cmd_vel_frame, which seems good for multi-robot purpose (I'm using it right now with dozens of pioneers on same environment).

Is this project abandoned? Hope my pull request is welcome.