NaN in columns produces TypeError
``` python
>>> pd.DataFrame([[1,2,3]], columns=[1.1,2.2,np.nan])
Traceback (most recent call last):
...
TypeError: ("'NoneType' object is not iterable", u'occurred at index 2.2')
```

Is this a bug or a feature? I don't see anything about that in the `v0.13.0.txt`, and it worked in 0.12:

``` python
>>> pd.DataFrame([[1,2,3]], columns=[1.1,2.2,np.nan])
    1.1   2.2  NaN
0     1     2     3
```
