Anti warp pad, hollow for faster tape down

This is an improved version of the [Anti Warp Pad](http://www.thingiverse.com/thing:1429413). The best way to use those is to tape them down, but that usually means waiting for the first layer to finish. The improved pads only print the outline which, depending slicer settings, gets printed first. Thus the taping down can be done in seconds instead of waiting several minutes.

The pads helps keep the corners from warping. Since the settings in Cura and others usually generate a bit more brim t