Add smyx-theme

smyx is a theme for emacs. Screenshots are available [in the repository](https://github.com/tacit7/smyx).

I am no direct contributor to this package, but just downloaded it, like it a lot and saw, that it wasn't on melpa, so I created this recipe, because I like to manage my dependencies with cask and melpa.

@tacit7 Just notifying the original author.