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/_list.scss

@@ -0,0 +1,19 @@
+// List Icons
+// -------------------------
+
+.#{$fa-css-prefix}-ul {
+  padding-left: 0;
+  margin-left: $fa-li-width;
+  list-style-type: none;
+  > li { position: relative; }
+}
+.#{$fa-css-prefix}-li {
+  position: absolute;
+  left: -$fa-li-width;
+  width: $fa-li-width;
+  top: (2em / 14);
+  text-align: center;
+  &.#{$fa-css-prefix}-lg {
+    left: -$fa-li-width + (4em / 14);
+  }
+}