dog-bone connector for wooden trains

A simple dog-bone (male-male) connector for Thomas- and Brio-compatible wooden train track.

This renders in "solid" and "spring" variations. The spring version is the original design but the now-default solid version looks better and actually uses quite a bit less filament (5-15% less for 25-15% infill).

To render this shape, you will need [tracklib.scad](http://www.thingiverse.com/thing:216915) installed in the same directory as this file, or the [dotscad/trains](https://github.com/dotsc