Use Jekyll native Sass/SCSS preprocessor

- Move .scss files to default `_sass` folder
- Configure Sass/SCSS in `_config.yml`
- Run .scss files through Autoprefixer
- Remove unnecessary npm scripts for building CSS
- Close #333

_config.yml

@@ -145,6 +145,12 @@ kramdown:
   enable_coderay: false
 
 
+# Sass/SCSS
+sass:
+  sass_dir: _sass
+  style: compressed # http://sass-lang.com/documentation/file.SASS_REFERENCE.html#output_style
+
+
 # Outputting
 permalink: /:categories/:title/
 paginate: 5 # amount of posts to show