DataFrame.to_hdf will write Unicode labels, but pd.read_hdf won't read them
I've been having a very frustrating problem where I could save a DataFrame with unique labels to HDF5, and load it again, and find that the loaded version had non-unique labels. It turns out this is because `pd.read_hdf` is replacing all labels containing non-ASCII characters with the empty string. For example, a row with the label `'café'` will have the label `''` when it is loaded again.

I looked at the bytes that were written in the HDF5 file, and confirmed that the proper UTF-8 text was in there, so the bug is in `read_hdf`.

Version information:

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.0.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-51-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.1
nose: 1.3.7
pip: 1.5.4
setuptools: 2.2
Cython: 0.23.3
numpy: 1.10.4
scipy: 0.16.1
statsmodels: 0.6.1
IPython: 4.0.0
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.4.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: 0.7.3
lxml: None
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
Jinja2: None
```
