Fixing gazebo_bridge gzmath include, minor bugfix, and small increment

Fix 1: (gazebo_bridge.cc)
#include math/gzmath.hh
should be
#include gazebo/math/gzmath.hh

This will enable compilation on a brand-new environment.

Fix 2: (gazebo_bridge.cc)
I have fixed the use of ros_odom_frame, which was being ignored. It was using hardcoded "/odom"...

Increment 1: (gazebo_bridge.cc)
I've added a param named ros_cmd_vel_frame, which seems good for multi-robot purpose (I'm using it right now with dozens of pioneers on same environment).

Increment 2:
I've changed CMakeLists.txt to consider gazebo as required... to enable compilation under gazebo4 over ros indigo.

Is this project abandoned? Hope my pull request is welcome.