improve platform independence (#296)

* add os.devnull for platform independence

fixes #288

* add test for soft_barcode

* open devnull as binary

* add version identifier to pickle

test/test_function_softbarcode.py

@@ -0,0 +1,16 @@
+#!/usr/bin/python
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import escpos.printer as printer
+import pytest
+
+
+def test_soft_barcode():
+    """just execute soft_barcode
+    """
+    instance = printer.Dummy()
+    instance.soft_barcode("ean8", "1234")
+