Mini Space Invaders Carriage (Eustathios)

This is a redesign of the [Space Invaders Carriage](http://www.thingiverse.com/thing:893849) for Eustathios printers. The main reason of the redesign was to make it smaller to increase usable build volume. Here's some of the changes:
* smaller size, XY travel is about 290mm² (still less than the original carriage)
* self-aligning bushings instead of graphite bushings (they use up less space)
* 40mm extruder fan instead of a 30mm fan (less noise)
* no longer supports mounting a direct drive bondtech

Like the previous version, it is convertible between a bowden configuration and a direct drive configuration using a Printrbot [Gear Head extruder](http://www.thingiverse.com/thing:1052966). But, instead of switching, I opted for a flying extruder with a short bowden tube (more about that setup in [this post](http://thrinter.com/cartesian-flying-extruder/)). The hotend is mounted quite a bit higher than with the stock Eustathios build, so the bed probably won't reach the nozzle wi