Right wheels rotate opposite direction to adjust to the real spur robot's config

[130s]

Screenshoted videos:

• Before: https://docs.google.com/file/d/0Bw_hbVS3wTLyMFdBcldFd2x2WW8/edit
• After: https://docs.google.com/file/d/0Bw_hbVS3wTLyMGM1X1FtWGpWTTg/edit

In the "After" video, it looks to me the wheel rotates in weird direction on Gazebo, which paradoxically might be the correct config on the real robot.

Or, do we need to find the correct config that works BOTH on Gazebo and the real robot?