Joint limits for the limited version could be set using arguments for the UR10
but not for the UR3 and UR5. Same lower and upper limit arguments are added to
the UR3 and UR5 xacro macros.

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