Kinder gentler LM8UU bearing holder with zip tie support

This part was done by me for the Reprap Wilson design, and I thought I would also release it separtely here in case others find it useful in other situations. This is based on the design by jridley; I've just made the screw holes on the sides slotted to give a little more wiggle room for when the hole's aren't drilled perfectly, and slightly increased the space for the bearing to sit in because they seemed a little too small on my prints.