Styler.render generates unquoted attribute values, which are illegal in XHTML
#### Code Sample, a copy-pastable example if possible

```python
>>> df = pd.DataFrame([[4,5]], columns=['A', 'B'])
>>> df.style.render()[355:405]
u'<th class="col_heading level0 col0" colspan=1>\n   '
```
#### Problem description

Including the generated html where valid XHTML is expected (for example, [in a Jekyll site using kramdown, the default markdown parser](http://stackoverflow.com/questions/41933713/pandas-dataframe-rendered-as-html-shows-up-weirdly-with-literal-tags-when-incl)) leads to weird behaviour.

#### Expected Output

The same thing, but with all attributes quoted, e.g. `colspan=1` => `colspan="1"`. As far as I can tell, there's no downside to always quoting attribute values.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 32
OS: Linux
OS-release: 3.13.0-107-generic
machine: i686
processor: i686
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 25.1.0
Cython: None
numpy: 1.12.0
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 1.5.7
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.1.3
html5lib: 0.999
httplib2: 0.8
apiclient: None
sqlalchemy: 0.8.0
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: None
pandas_datareader: None
</details>
