added polyhedron() implementation

Hi I was having a play with scad-clj and wanted to use the polyhedron() function but it seems that it's not implemented. There may be a good reason why, but I'm very new to scad-clj/OpenScad so not aware of potential issues.

I've added it and it works for me, and I thought you may be able to use it. I hope this works okay, I've never done a pull request before.

Regards,
Jules