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/_rotated-flipped.scss

@@ -0,0 +1,20 @@
+// Rotated & Flipped Icons
+// -------------------------
+
+.#{$fa-css-prefix}-rotate-90  { @include fa-icon-rotate(90deg, 1);  }
+.#{$fa-css-prefix}-rotate-180 { @include fa-icon-rotate(180deg, 2); }
+.#{$fa-css-prefix}-rotate-270 { @include fa-icon-rotate(270deg, 3); }
+
+.#{$fa-css-prefix}-flip-horizontal { @include fa-icon-flip(-1, 1, 0); }
+.#{$fa-css-prefix}-flip-vertical   { @include fa-icon-flip(1, -1, 2); }
+
+// Hook for IE8-9
+// -------------------------
+
+:root .#{$fa-css-prefix}-rotate-90,
+:root .#{$fa-css-prefix}-rotate-180,
+:root .#{$fa-css-prefix}-rotate-270,
+:root .#{$fa-css-prefix}-flip-horizontal,
+:root .#{$fa-css-prefix}-flip-vertical {
+  filter: none;
+}