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/font-awesome/_path.scss

@@ -0,0 +1,15 @@
+/* FONT PATH
+ * -------------------------- */
+
+@font-face {
+  font-family: 'FontAwesome';
+  src: url('#{$fa-font-path}/fontawesome-webfont.eot?v=#{$fa-version}');
+  src: url('#{$fa-font-path}/fontawesome-webfont.eot?#iefix&v=#{$fa-version}') format('embedded-opentype'),
+    url('#{$fa-font-path}/fontawesome-webfont.woff2?v=#{$fa-version}') format('woff2'),
+    url('#{$fa-font-path}/fontawesome-webfont.woff?v=#{$fa-version}') format('woff'),
+    url('#{$fa-font-path}/fontawesome-webfont.ttf?v=#{$fa-version}') format('truetype'),
+    url('#{$fa-font-path}/fontawesome-webfont.svg?v=#{$fa-version}#fontawesomeregular') format('svg');
+//  src: url('#{$fa-font-path}/FontAwesome.otf') format('opentype'); // used when developing fonts
+  font-weight: normal;
+  font-style: normal;
+}