First attempt at centering images and QRs (#250)

This was tested on ZJ-5890 with success. By default centering is
deactivated for backward compatibility. Trying to center a QR code in
native mode will raise an exception as we do not know ATM if the native
rendering is centered by default or not.

* Added basic tests for center feature

* Check image size before centering

src/escpos/image.py

@@ -115,3 +115,19 @@ class EscposImage(object):
             box = (left, upper, right, lower)
             fragments.append(self.img_original.crop(box))
         return fragments
+
+    def center(self, max_width):
+        """In-place image centering
+
+        :param: Maximum width in order to deduce x offset for centering
+        :return: None
+        """
+        old_width, height = self._im.size
+        new_size = (max_width, height)
+
+        new_im = Image.new("1", new_size)
+        paste_x = int((max_width - old_width) / 2)
+
+        new_im.paste(self._im, (paste_x, 0))
+
+        self._im = new_im