fillna not accepting a None value
In a DataFrame, it's not possible to use `fillna` to replace NaN values by `None`:

``` python
df = pandas.DataFrame([np.nan, 42])
df.fillna(value=None)
```

returns a `ValueError` exception (_ValueError: must specify a fill method or value_). Any other value is of course accepted.

I'm not sure if it's a good idea to call `fillna` for that, but it should at least be explained in the documentation that `None` cannot be used.
