Ensure QR codes have a border large enough for successful scan

(The QR code spec requires a border at least 4*box_size thick but we can't
 just set border=16 because that results in a QR code more than 255px tall
 and I'm not yet ready to use fullimage() as a backend for it)

escpos/escpos.py

@@ -152,7 +152,10 @@ def qr(self, text):
         qr_img = qr_code.make_image()
         # Convert the RGB image in printable image
         im = qr_img._img.convert("RGB")
+        self.text('\n')
+        self.set(align='center')
         self.image(im)
+        self.text('\n')
 
     def barcode(self, code, bc, height, width, pos, font):
         """ Print Barcode """