smooth stubby screw driver handle

refinement of [kheibert's](http://www.thingiverse.com/khiebert) stubby screw driver handle. I took it into meshmixer and smoothed out all the sharp edges. I also added a slight taper inside the hole, so that the bit will be held in more tightly (although if you push too far, you may need pliers to pull it back out)

Prints in about 25 minutes on a Rep2 at medium quality defaults, no raft or supports.