Ensure QR codes have a border large enough (#235)

*  Ensure QR codes have a border large enough
(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)
This fix was originally commited by Stephan Sokolow on 2014-05-22
* Let the user print stuff using qr example
* fix tests

examples/qr_code.py

@@ -0,0 +1,19 @@
+import sys
+
+from escpos.printer import Usb
+
+
+def usage():
+    print("usage: qr_code.py <content>")
+
+
+if __name__ == '__main__':
+    if len(sys.argv) != 2:
+        usage()
+        sys.exit(1)
+
+    content = sys.argv[1]
+
+    # Adapt to your needs
+    p = Usb(0x0416, 0x5011, profile="POS-5890")
+    p.qr(content)