Add shop's basic views, urls and templates.
Fix store ad's url.
Fix header hrefs.
Accept category_id at store_front. Use lowercase with undersore for function names.
Use category prefix for all category.
Add url for retrieving store_items.
Splite store_front into store_home and store_category.
Implement store_items API for single category.
Add template for store_category view.
Add page argument for store_items view.
Use data-source tag to load data for artical. Move item related scripts into item.html.
Implement paging for store_items.
Simplify store_items logic.
Fix on_sale and sales_price display.
Add API for popular_items and onsale_items.
Add item_detail template for displaying item detail.
Update store_home template.