TF frames for joints with assymetric joint limits are not visualised correctly

[gavanderhoorn]

As reported in #69, from @akgoins in #70 (comment):

The problem with the frames not showing up correctly for the PR2 has to do with the joint limits. If the joint limits are (-N,N), then the robot is visualized with the joint at 0. If the joint limit is (0,N) or (-N, 0), then the robot is visualized with the joint set to midscale. This can be seen by running roslaunch urdf_tutorial urdf_display model:=<dir>/pr2.urdf gui:=true. The gui shows some of the joint values at a non-zero start state. The TF publisher currently assumes that all joint values are set to zero. We will need to add some sort of mechanism that can set each joint to its appropriate value for correct visualization.