decimated many arm collision meshes (stl files) to speed up moveit.

This is in response to https://github.com/ros-planning/moveit_ros/issues/380 where I found that the startup time for move_group (in debug mode, it is true) was about 30 seconds and almost all of that time was spent simplifying meshes.

This pull request is the result of me opening blender and importing each of the changed .stl meshes, decimating them with the decimate tool to the lowest ratio that seemed like it still looked "pretty good" to my eye, and then exporting the .stl file back over the original.

I believe each of these meshes is only used for the *collision* data for the PR2's urdf, which means this should not change the typical appearance of the PR2 in rviz at all.  The visual models use collada (.dae) files and those have not been changed here.

This set of changes reduces the relevant part of my (debug mode) moveit startup time from 30 seconds to about 0.5 seconds.

Here is what the PR2 collision meshes look like before this change:
![original-pr2](https://f.cloud.github.com/assets/1562730/1636427/c184dcdc-57cd-11e3-9366-c91196594fcb.png)

And here is what the PR2 collision meshes look like after this change:
![decimated-pr2](https://f.cloud.github.com/assets/1562730/1636428/c67b81fa-57cd-11e3-8fb9-a9b32d33bb67.png)