Update license, add buttons scss.

All I really added was the `$pureButtonColor` variable. I'm assuming this would work best, similar to how [bootstrap-sass](https://github.com/thomas-mcdonald/bootstrap-sass) does it. 

I went with the SCSS syntax since it only requires a cut and paste with added variables.


Where you can first define the variables before calling `@import 'bootstrap';`, which will cause them to be overridden in the imported stylesheets. That way one can readily `@extend` and redefine the Pure color schemes, padding, etc. on the fly without having to use their UI for customization.

Is this on par with what you were intending/thinking? I didn't go farther with this tonight since I wanted to make sure we had the same idea.

Also, this PR adds .DS_Store to the gitignore.