Initial commit for _sass.

Props @michaelArestad @hugobeta, and @jacklenox.

* Give prefix to variable files. This avoids confusion with files of similar names like typography. It also helps with the visual recognition over what file does what.
* Set up mixins-master.
* Setting foundation for encouraging in larger projects use of more than one mixing file. This creates a master mixins file that we can then bring subs into if the project grows.
* Fixed line breaks, spacing and new lines. Props @gregrickaby.
* Nest widget select. Props @gregrickaby.
* Make variables be variables-site. Makes this the global variables folder. People can then use sub folders in sections for local only variables.
* Change content directory to be site directory.
* Remove zip. Revert `style.css` to not be auto generated version.
* Fixes rem size issue on body font.
* Fixes for differences between `_s` and `_sass`. Fixed font sizes, added in display block to classes so they compile and the theme check
passes.
* Remove text domain and clean up css.
_s: Bring back closing curly brace.

Erroneously removed while trying to sort out a merge conflict, when I
squashed `_sass` commits.
Merge pull request #550 from Automattic/_sass

Brings sass into _s

Sass now comes with _s - you can simply check an option on underscores.me and get a version with Sass.

Props to: @gregrickaby @bradp @hugobaeta @obenland @sabreuse @MichaelArestad @jacklenox @karmatosed