pd.crosstab, categorical data and missing instances
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
foo = pd.Categorical(['a', 'b'], categories=['a', 'b', 'c'])
bar = pd.Categorical(['d', 'e'], categories=['d', 'e', 'f'])
pd.crosstab(foo, bar)

col_0  d  e
row_0      
a      1  0
b      0  1
c      0  0

```
#### Problem description

This is from the documentation: 

> Any input passed containing Categorical data will have **all** of its categories included in the cross-tabulation, even if the actual data does not contain any instances of a particular category.

However, *f* is not included in the table while *c* is.

Please let me know if this is in fact a bug, then I will be glad to write give writing a patch a try.

Thanks a lot in advance!

#### Expected Output

col_0  d  e  f
row_0
a      1  0  0
b      0  1  0
c      0  0  0

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-49-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: 2.8.7
pip: 8.1.1
setuptools: 20.7.0
Cython: None
numpy: 1.12.1
scipy: 0.17.0
xarray: None
IPython: None
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 1.5.1
openpyxl: 2.3.0
xlrd: 0.9.4
xlwt: 0.7.5
xlsxwriter: None
lxml: 3.5.0
bs4: 4.4.1
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
