Add carton recipe.

Hey,

Not too long ago I did a pull request to add Carton, which we decided to not merge (https://github.com/milkypostman/melpa/pull/328). I have now realized that Carton should be available as an ELPA package for a couple of reasons. For one, it will add ELPA sources specified in the `Carton` file. I've also been requested to add Carton by others.

Thanks