add scad-mode recipe

i'm getting into emacs and OpenSCAD, and i'd love for the scad-mode package to be available through MELPA. 

seeing as i'm new to emacs and i've had no involvement in this package's development, i hope i've done this right, but i'd appreciate any feedback.

this package adds support for the OpenSCAD language, and the repo is at [openscad/openscad](http://github.com/openscad/openscad)