enable the tf_prefix parameter

`tf_prefix` is being [deprecated](http://www.ros.org/wiki/hydro/Migration#tf2.2BAC8-Migration.Removal_of_support_for_tf_prefix) in ROS Hydro. I would still like to use the `tf_prefix` parameter http://www.ros.org/wiki/robot_state_publisher#Parameters to distinguish between different robots in multi-robot systems.

It seems that the robot_state_publisher code is not using either `tf::resolve` or reading in the `tf_prefix`, so I am a bit confused as to how this was was working pre-Hydro. I remember running some tests a few months ago that worked.

This patch reads the parameter explicitly and sets it using `tf::resolve`. Using this patch, I can run multiple robots in Gazebo without tf tree collisions.