Variable labels as a dataframe field
I use both Stata and Pandas. Many Stata users save variable labels to describe the columns in a clearer way than the names. Running this in Stata

```
sysuse auto.dta
describe
```

gives something like

| variable name | storage type | variable label |
| --- | --- | --- |
| make | str18 | Make and Model |
| price | int | Price |
| mpg | int | Mileage (mpg) |

For me (maybe for others too) it would be useful to have an optional field in a DataFrame with a column label dictionary. The keys would be the columns (not necessarily all of them) and the values the string labels.

This is used in the pandas.io.stata.StataReader field `variable_labels`(see [the docs](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.io.stata.StataReader.variable_labels.html)], that allows you to import these labels when one reads in a Stata `.dta` file.

I know I could just carry around a dictionary with this information, but I think it's cleaner and less error prone to set it and save it within a DataFrame.

Additionally, storing this would allow doing a cycle on Stata/Pandas without loss of information, since the `to_stata` would check if this field exists. (`to_stata` might already have the option to pass the `variable_labels` dictionary as an option, but I didn't see it documented at least)

My coding prowess is quite limited, but I'd be happy to at least write test code and help out if somebody starts out.
