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/_font-awesome.scss

@@ -0,0 +1,18 @@
+/*!
+ *  Font Awesome 4.6.3 by @davegandy - http://fontawesome.io - @fontawesome
+ *  License - http://fontawesome.io/license (Font: SIL OFL 1.1, CSS: MIT License)
+ */
+
+@import "variables";
+@import "mixins";
+@import "path";
+@import "core";
+@import "larger";
+@import "fixed-width";
+@import "list";
+@import "bordered-pulled";
+@import "animated";
+@import "rotated-flipped";
+@import "stacked";
+@import "icons";
+@import "screen-reader";