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

package.json

@@ -21,22 +21,13 @@
     "node": ">= 0.10.0"
   },
   "devDependencies": {
-    "autoprefixer": "^6.3.5",
-    "node-sass": "^3.4.2",
     "npm-run-all": "^1.7.0",
     "onchange": "^2.2.0",
-    "postcss-cli": "^2.5.1",
     "uglify-js": "^2.6.1"
   },
   "scripts": {
-    "scss": "node-sass --output-style compressed -o assets/css assets/_scss",
-    "autoprefixer": "postcss -u autoprefixer -b \">, 5%, ie 9\" -r assets/css/*",
     "uglify": "uglifyjs assets/js/vendor/jquery/jquery-1.12.4.min.js assets/js/plugins/jquery.fitvids.js assets/js/plugins/jquery.greedy-navigation.js assets/js/plugins/jquery.magnific-popup.js assets/js/plugins/jquery.smooth-scroll.min.js assets/js/plugins/stickyfill.min.js assets/js/_main.js -c -m -o assets/js/main.min.js",
-    "watch:css": "onchange \"assets/_scss/**/*.scss\" -- npm run build:css",
     "watch:js": "onchange \"assets/js/**/*.js\" -e \"assets/js/main.min.js\" -- npm run build:js",
-    "watch:all": "npm-run-all --parallel watch:css watch:js",
-    "build:css": "npm run scss && npm run autoprefixer",
-    "build:js": "npm run uglify",
-    "build:all": "npm run build:css && npm run build:js"
+    "build:js": "npm run uglify"
   }
 }