DOC:  min_itemsize for HDFStore append for encoded strings
I'm confused about how to preset min_itemsizes for appending to an HDFStore. Say DataFrame a and b in the MWE below is user-provided, so it can contain any character and the encoding is unknown. Appending a works, but appending b fails even though:

```python
In [4]: len('香')
Out[4]: 1
```
So far I simply used str.len().max() on the string columns to the the numbers for min_itemsize, but this does not work in the example here. This MWE is of course simplified, but I guess I'm wondering:

- how does pytables come up with the string length?
- how should I determine the string length? Considering the encoding is unknown, but pytables assumes some encoding / pytables converts the strings to some other object?

In this toy example I could encode the string as utf-8 to get the correct length, but this isn't a general approach:

```python
In [5]: len('香'.encode('utf-8'))
Out[5]: 3
```

#### MWE:
```python
import pandas as pd
                                                      
a = pd.DataFrame([['a', 'b']], columns = ['A', 'B'])
b = pd.DataFrame([['香', 'b']], columns = ['A', 'B'])

store = pd.HDFStore('/tmp/tmpstore')

store.append('df', a, min_itemsizes={'A': 1, 'B': 1})
store.append('df', b, min_itemsizes={'A': 1, 'B': 1}) # fails

```

#### Expected Output
ValueError: Trying to store a string with len [3] in [values_block_0] column but
this column has a limit of [1]!
Consider using min_itemsize to preset the sizes on these columns
Closing remaining open files:/tmp/tmpstore...done

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

<details>INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.28-2-MANJARO
machine: x86_64
processor: 
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_DE.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.0
nose: 1.3.7
pip: 8.1.2
setuptools: 27.2.0
Cython: 0.23.5
numpy: 1.11.2
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: 1.4.8
patsy: None
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.1.3
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: None
pandas_datareader: None
</details>
