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

_sass/vendor/susy/_susyone.scss

@@ -0,0 +1,4 @@
+// Susy
+// ====
+
+@import 'susy/language/susyone';