Add sass compiler

In the thread https://github.com/linnovate/mean/issues/676#issuecomment-65501511, a CSS preprocessor was discussed.  I set this up in my own project so though I'd submit a PR.

To use, add scss files to the package/public/assets/sass folder and they'll automatically get compiled.  (for example, package/public/assets/sass/myfile.scss gets compiled to package/public/assets/css/myfile.compiled.css)

Under the hood it uses grunt-contrib-sass.