Add io.StringIO subclass for writing gpg process results to a string.

* These .write()s need to be forced to unicode for StringIO to handle them
   correctly.

test_gnupg.py

@@ -7,6 +7,7 @@ Copyright © 2008-2013 Vinay Sajip. All rights reserved.
 """
 import doctest
 import logging
+import io
 import os.path
 import os
 import shutil
@@ -89,6 +90,13 @@ def compare_keys(k1, k2):
     del k2[1]
     return k1 != k2
 
+class ResultStringIO(io.StringIO):
+    def __init__(self):
+        super(self, io.StringIO).__init__()
+
+    def write(self, data):
+        super(self, io.StringIO).write(unicode(data))
+
 class GPGTestCase(unittest.TestCase):
     def setUp(self):
         hd = os.path.join(os.getcwd(), 'keys')