[FIX] point_of_sale: Print receipt on Safari

As explained on stackoverflow with
https://stackoverflow.com/questions/23626998/safari-print-issue-with-javascript-window-print
There is an issue with javascript window.print() on Safari

Steps to reproduce the bug:
When clicking on button "Print receipt" in the point of sale, nothing happened.

opw:1858205

addons/point_of_sale/static/src/js/screens.js

@@ -1547,7 +1547,12 @@ var ReceiptScreenWidget = ScreenWidget.extend({
         };
     },
     print_web: function() {
-        window.print();
+        if($.browser.safari){
+            document.execCommand('print', false, null);
+        }
+        else{
+            window.print();
+        }
         this.pos.get_order()._printed = true;
     },
     print_xml: function() {