Changes to WC_Product_Builder Class

* Renamed session methods (e.g. clear_session -> session_clear)
* New method sorting
* New method "include_template"
* Removed methods "category_options" and "category_tablist" (replaced by
include_template)
* Removed methods "update_product" and "update_product_price"
New assets directory (for css, js, img)

* Created "assets" directory
* Moved CSS to assets dir
* Edited main.css for use with SASS to main.sass
* Added jQuery and main.js
New Frontend Templates

New frontend templates for product review
Changes to backend HTML / frontend CSS insertion
Updates to Frontend Templates