Feature/clear content in dummy printer (#271)

Add Function to Dummy Printer for Clearing Buffer

If you are using the dummy printer, you may want to use the printer
again after sending the output to a physical printer.
This method empties the list of the output buffer.

test/test_function_dummy_clear.py

@@ -0,0 +1,8 @@
+from nose.tools import assert_raises
+from escpos.printer import Dummy
+
+def test_printer_dummy_clear():
+    printer = Dummy()
+    printer.text("Hello")
+    printer.clear()
+    assert(printer.output == b'')