NaN not included in col.cat.categories
I've searched for issues, but haven't found a similar one - apologies if there are.

Current behaviour:

```
import pandas as pd
df = pd.DataFrame({'col': ['a', None]})
df['col'] = df.col.astype('category')
dict(enumerate(df.col.cat.categories))
# {0: 'a'}
```

Expected behaviour would be to include the `NaN` e.g.

```
{-1: NaN, 0: 'a'} 
```

This would match the behaviour of `df.col.cat.codes` and `df.col.unique()`, both of which include the `NaN`.

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.21.0
pytest: None
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: None
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
