Prusa v2 X-Y Calibrating Aid

This is the x-y calibrating aid/pointer from my earlier upload, but modified for use with a Prusa v2 with linear bearings (I just completed my prusa v2 kit from Makerfarm and realized that there is no way to mount the x-y pointer I made for my printrbot).

Some improvements I made are:

1. Held via friction (using the linear bearing as a retainer) so no separate clip needed to hold it in place.
2. Two pointers at 90 deg on the tip so you can do the x and y calibration without having to r