Backport PR #4163: Fix for incorrect default encoding on Windows.

Whilst trying out rendering notebooks in a flask app under Apache on Windows I got the below error when simply trying to import `SlidesExporter`
```python
mod_wsgi (pid=6260): Exception occurred processing WSGI script 'flask_test.wsgi'.
Traceback (most recent call last):
  File "flask_test.py", line 81, in render_notebook
    from IPython.nbconvert.exporters import SlidesExporter
  File "c:\\dev\\code\\ipython\\IPython\\__init__.py", line 47, in <module>
    from .terminal.embed import embed
  File "c:\\dev\\code\\ipython\\IPython\\terminal\\embed.py", line 32, in <module>
    from IPython.terminal.interactiveshell import TerminalInteractiveShell
  File "c:\\dev\\code\\ipython\\IPython\\terminal\\interactiveshell.py", line 25, in <module>
    from IPython.core.interactiveshell import InteractiveShell, InteractiveShellABC
  File "c:\\dev\\code\\ipython\\IPython\\core\\interactiveshell.py", line 59, in <module>
    from IPython.core.prompts import PromptManager
  File "c:\\dev\\code\\ipython\\IPython\\core\\prompts.py", line 138, in <module>
    HOME = py3compat.str_to_unicode(os.environ.get("HOME","//////:::::ZZZZZ,,,~~~"))
  File "c:\\dev\\code\\ipython\\IPython\\utils\\py3compat.py", line 18, in decode
    return s.decode(encoding, "replace")
LookupError: unknown encoding: cp0
```
A little bit of [googling](http://bugs.python.org/issue6501) suggests that Windows returns 'cp0' to indicate there is no code page. This fix simply looks for this invalid value and replaces it with something valid. With this change it works for me.

IPython/utils/encoding.py

@@ -1,56 +1,67
 # coding: utf-8
 """
 Utilities for dealing with text encodings
 """
 
 #-----------------------------------------------------------------------------
 #  Copyright (C) 2008-2012  The IPython Development Team
 #
 #  Distributed under the terms of the BSD License.  The full license is in
 #  the file COPYING, distributed as part of this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 import sys
 import locale
+import warnings
 
 # to deal with the possibility of sys.std* not being a stream at all
 def get_stream_enc(stream, default=None):
     """Return the given stream's encoding or a default.
 
     There are cases where ``sys.std*`` might not actually be a stream, so
     check for the encoding attribute prior to returning it, and return
     a default if it doesn't exist or evaluates as False. ``default``
     is None if not provided.
     """
     if not hasattr(stream, 'encoding') or not stream.encoding:
         return default
     else:
         return stream.encoding
 
 # Less conservative replacement for sys.getdefaultencoding, that will try
 # to match the environment.
 # Defined here as central function, so if we find better choices, we
 # won't need to make changes all over IPython.
 def getdefaultencoding():
     """Return IPython's guess for the default encoding for bytes as text.
 
     Asks for stdin.encoding first, to match the calling Terminal, but that
     is often None for subprocesses.  Fall back on locale.getpreferredencoding()
     which should be a sensible platform default (that respects LANG environment),
     and finally to sys.getdefaultencoding() which is the most conservative option,
     and usually ASCII.
     """
     enc = get_stream_enc(sys.stdin)
     if not enc or enc=='ascii':
         try:
             # There are reports of getpreferredencoding raising errors
             # in some cases, which may well be fixed, but let's be conservative here.
             enc = locale.getpreferredencoding()
         except Exception:
             pass
-    return enc or sys.getdefaultencoding()
+    enc = enc or sys.getdefaultencoding()
+    # On windows `cp0` can be returned to indicate that there is no code page.
+    # Since cp0 is an invalid encoding return instead cp1252 which is the
+    # Western European default.
+    if enc == 'cp0':
+        warnings.warn(
+            "Invalid code page cp0 detected - using cp1252 instead."
+            "If cp1252 is incorrect please ensure a valid code page "
+            "is defined for the process.", RuntimeWarning)
+        return 'cp1252'
+    return enc
 
 DEFAULT_ENCODING = getdefaultencoding()