Printing None and NaN values in Pandas dataframe produces confusing results
Printing a dataframe where a variable contains None values produces confusing results. Large dataframes are automatically split to print to screen. If all the values on one side of the splits are None, they are actually displayed as NaN. This can be demonstrated with the following code.

```
import pandas as pd
tempDF = pd.DataFrame({'id':    np.arange(1,11),
                       'text':  ['some words'] + [None]*9})
print('Full database\n-------------\n',tempDF,'\n')
with pd.option_context('display.max_rows', 8, 'display.max_columns', 3):
    print('Split database\n--------------\n',tempDF,'\n')
```

This produces the following output:

```
Full database
-------------

    id        text
0   1  some words
1   2        None
2   3        None
3   4        None
4   5        None
5   6        None
6   7        None
7   8        None
8   9        None
9  10        None 

Split database
--------------
     id        text
0    1  some words
1    2        None
2    3        None
3    4        None
..  ..         ...
6    7         NaN
7    8         NaN
8    9         NaN
9   10         NaN

[10 rows x 2 columns] 
```

Above the split, the variable 'text' has one cell which has a genuine string ('some words'). All the None values on that side of the split are correctly displayed as 'None'. However, on the bottom part of the split, all the cells contain None values but are confusingly displayed as Nan.

Expected behaviour: All None values should be displayed as 'None' rather than 'NaN'.

```
pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.1.final.0
python-bits: 64
OS: Darwin
OS-release: 15.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8

pandas: 0.17.1
nose: None
pip: 1.5.6
setuptools: 3.6
Cython: None
numpy: 1.10.2
scipy: 0.16.1
statsmodels: None
IPython: 4.0.1
sphinx: None
patsy: None
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.0
openpyxl: 2.3.2
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
Jinja2: 2.8
```
