created the wc_get_product_types() function
initial create product API method
fixed a error when get the thumbnail by API

Fixed the error:

PHP Notice: Trying to get property of non-object in wp-includes/post-template.php on line 29
created methods to save product images by API
improved the save product images methods in product API
added @kloon's WC_API_Products::save_product_meta() method
added delete method in products API
fixed the create producs method in products API
fixed the save product method in API
added PUT method in product API
fixed/improved the PUT method in product API
added method to save product attributes in products API
improved the product price in product API handler
added all product options for save in WC_API_Products::save_product_meta() method
fixed the downloads method
created API methods to save product categories and tags
added method to save the variations in product API
added new product API actions
fixed some coding standards
Merge pull request #5745 from claudiosmweb/4164-feature-products-api

Products API - PUT/POST/DELETE