append / concat category  with different categories fails in HDF5


Hi,

just a remark, I guess it must be fairly complicated to replicate the behaviour from pandas ( appending categoricals, with different categories)  into HDF5.

Currently this fails 

```
pd1.to_hdf(store_file,"/ISE_nombre_de_tabla",format="table",append=True)
pd2..to_hdf(store_file,"/ISE_nombre_de_tabla",format="table",append=True)

.
.
.

1698             if new_metadata is not None and cur_metadata is not None \
1699                     and not array_equivalent(new_metadata, cur_metadata):
-> 1700                 raise ValueError("cannot append a categorical with "
1701                                  "different categories to the existing")
1702 

ValueError: cannot append a categorical with different categories to the existing
```

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 61 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.19.1
nose: 1.3.7
pip: 8.1.2
setuptools: 23.0.0
Cython: 0.24
numpy: 1.11.1
scipy: 0.17.1
statsmodels: 0.6.1
xarray: None
IPython: 4.2.0
sphinx: 1.3.1
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.0
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.2
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.40.0
pandas_datareader: None
</details>