Splines for OpenSCAD

I was working on a project in OpenSCAD that needed smooth curves. I was excited to find some [Bezier curve libraries](http://www.thingiverse.com/thing:8443), but as I used them I remembered why awkward thing about Bezier curves: they're hard to design with if you want precise control. You move a control point, and it pulls the curve one way, but it also affects other areas of the curve, so you have to nudge adjacent control points the other way to compensate, and then repeat forever.

What I wanted was to tell OpenSCAD exactly where I wanted the curve to go, with a few points, and have it make a smooth curve passing between those points. That's a [spline](https://en.wikipedia.org/wiki/Spline_(mathematics)). I couldn't find any spline libraries for OpenSCAD. So I made this one.

The main module, `spline.scad`, is directly usable with no other dependencies. Grab the zip file instead if you want to look at the examples, as they're dependent on a couple of other modules.

#### Functi