ENH: Support Stata variable labels
xref https://github.com/pydata/pandas/issues/13535#issuecomment-229652201

cc @frehoy

> It seems that is is almost implemented here.
> 
> See
> https://github.com/pydata/pandas/blob/master/pandas/io/stata.py#L2057
> 
> and
> 
> https://github.com/pydata/pandas/blob/master/pandas/io/stata.py#L2134
> 
> As you can see, this always passes the default None and so no labels are written.
> 
> Mostly needs an external interface and a tiny amount of wiring up. And testing, esp that the labels can be read into Stata.

A possible API

``` python
import pandas as pd
d = {'one' : [1., 2., 3., 4.]}
df = pd.DataFrame(d)
labdict = {'one': 'foo'}
df.to_stata('test.dta', write_index=False, variab_labels=labdict)
```
