Micro-revert of #251.

If dependent xacros include the ur* urdfs, but don't use Jade+ xacro, 'pi'
will be undefined otherwise.

This is more in-line with naming of existing joint tags.

Note that 'base' is essentially 'base_link' but rotated by 180
degrees over the Z-axis. This is necessary as the visual and
collision geometries appear to also have their origins rotated
180 degrees wrt the real robot.

'tool0' is similar to 'ee_link', but with its orientation such
that it coincides with an all-zeros TCP setting on the UR
controller. Users are expected to attach their own TCP frames
to this frame, instead of updating it (see also [1]).

[1] http://wiki.ros.org/Industrial/Tutorials/WorkingWithRosIndustrialRobotSupportPackages#Standardised_links_.2BAC8_frames

https://github.com/ros-planning/moveit_core/issues/158