Integrates Ridgeback URDF with Baxter's Xacro URDF

[rethink-imcmahon]

• Adds a ridgeback_baxter_world.launch to demonstrate how to launch
  the Ridgeback with Baxter in Gazebo (this needs some Ridgeback tweaks)
• Adds a pedestal stand segment and DAE's for combining the two platforms
• Makes Baxter's Torso the parent of the Ridgeback for integrating with
  Baxter's onboard mutable robot state publisher
• Makes Baxter model optional to allow for publishing the
  Ridgeback-Pedestal combo as a URDF fragment for the real Baxter:

  $ rosrun xacro xacro.py --inorder
    `rospack find ridgeback_baxter_description`/urdf/ridgeback_baxter.urdf.xacro
     with_baxter:=false  > ridgeback_baxter.xml
  # After making ridgeback_baxter.yaml with the following values:
  # time:
  #   secs: 1449803750
  #   nsecs: 232993
  # link: 'torso'
  # joint: 'pedestal_fixed'
  # urdf: xml from ridgeback_baxter.xml
  $ rostopic pub /robot/urdf baxter_core_msgs/URDFConfiguration -f ridgeback_baxter.yaml -l
  # The real robot will replace its pedestal with the URDF fragment that you just created
  # (you will likely then want to publish your wheel joint states to /robot/joint_states)

• Enjoy the marvels of your new mobile Baxter!

[rethink-imcmahon]

This PR requires the release-1.2 branches for all of the Baxter repos. They will be merged into master late this week or early next week (whenever QA gives me the green light).

[rethink-imcmahon]

RViz attached to a real Baxter with the Ridgeback added:

[rethink-imcmahon]

I am actually not sure if this is the right mass or inertia for the pedestal stand...