use base_country for tax calculations in manually created orders

problem: 1.create manually an order. 2. add an item and click on the "Calc taxes" button. the js alert says that it will use the base country if no billing/shipping country was specified in the address. unfortunatly this functionality wasn't yet programmed in the javascript. 

solution: added  "woocommerce_writepanel_params.base_country" to the params. then included the caluclation code in the js file.

note: i'm not sure if this has any influence on other calculations. thanks for checking.