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/_animated.scss

@@ -0,0 +1,34 @@
+// Spinning Icons
+// --------------------------
+
+.#{$fa-css-prefix}-spin {
+  -webkit-animation: fa-spin 2s infinite linear;
+          animation: fa-spin 2s infinite linear;
+}
+
+.#{$fa-css-prefix}-pulse {
+  -webkit-animation: fa-spin 1s infinite steps(8);
+          animation: fa-spin 1s infinite steps(8);
+}
+
+@-webkit-keyframes fa-spin {
+  0% {
+    -webkit-transform: rotate(0deg);
+            transform: rotate(0deg);
+  }
+  100% {
+    -webkit-transform: rotate(359deg);
+            transform: rotate(359deg);
+  }
+}
+
+@keyframes fa-spin {
+  0% {
+    -webkit-transform: rotate(0deg);
+            transform: rotate(0deg);
+  }
+  100% {
+    -webkit-transform: rotate(359deg);
+            transform: rotate(359deg);
+  }
+}