Grocery Getter Manifold Fix

I found STIdeas' Grocery Getter very useful, but one thing that really bothered me was that it was non-manifold. Some of you may have encountered this issue when trying to add helper discs to the model in Markerbot Desktop, where void spaces would be created in the overlapping areas.

This manifold version has that problem resolved.

All credits to STIdeas at http://www.thingiverse.com/thing:22396