[pandas/io/stata.py] Importing option for importing specified columns from stata .dta files
It would be great if a `columns` option could be added to the Stata Reader. Some `dta` files can be large and I would like to import a few specified columns. Current I am importing the entire file and returning a selection such as:

``` python
data = pd.read_stata(fn)[['year', 'exports']]
```

I would propose `pd.read_stata(fn, columns=['year', 'exports'])`
