add with-statement

* with imported from mosquito (adaption of the code in 86a3e8dfb257e3af0ec141ee5d97717501d6e945 by @mosquito)
* encoding of file to utf-8
* added rudimentary test for with
* implement close function for all printers

test/test_with_statement.py

@@ -0,0 +1,24 @@
+#!/usr/bin/python
+"""test the facility which enables usage of the with-statement
+
+:author: `Patrick Kanzler <patrick.kanzler@fablab.fau.de>`_
+:organization: `python-escpos <https://github.com/python-escpos>`_
+:copyright: Copyright (c) 2016 `python-escpos <https://github.com/python-escpos>`_
+:license: GNU GPL v3
+"""
+
+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 escpos.escpos as escpos
+import os
+
+def test_with_statement():
+    """Use with statement"""
+    dummy_printer = printer.Dummy()
+    with escpos.EscposIO(dummy_printer) as p:
+            p.writelines('Some text.\n')
+    # TODO extend these tests as they don't really do anything at the moment