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.