new Item tag/category relations and APIs

[yiyione]

new relations:
ItemTagRelation / ItemCategoryRelation

New APIs

router
.route('/items/:itemId/tag')
.get(token.checkAuthAndGetUserInfo, itemController.listTags); // list item's all tags

router
.route('/items/:itemId/tag/:tagId')
.post(token.checkAuthAndGetUserInfo, itemController.addTag) // add tag to item
.delete(token.checkAuthAndGetUserInfo, itemController.deleteTag); // delete tag from item

router
.route('/items/:itemId/category')
.get(token.checkAuthAndGetUserInfo, itemController.listCategories); // list item's all categories

router
.route('/items/:itemId/category/:categoryId')
.post(token.checkAuthAndGetUserInfo, itemController.addCategory) // add category to item
.delete(token.checkAuthAndGetUserInfo, itemController.deleteCategory); // delete category from item

[hippogr]