Changes/additions to gruntfile.js

Did my best to get pretty close to WP CSS code standards, but haven’t
figured out how to retain newlines… Yet.

Autoprefixer has been added.

You will need to run `npm install` again. Grunt watch is now 100%
beefier.
Updated sass paths.
Removed jsbeautify

It was causing spacing issues rather than solving them.
Merge pull request #665 from Automattic/sassier-grunt

Sassier grunt