Export and import dataframe with tuples headers
Hello,

I have another issue with tuples. If you export and import a DataFrame to csv the tuples headers are transformed to string.

Here is an example of test:

``` python
import pandas as pd
from StringIO import StringIO
df_out = pd.DataFrame()
df_out['A'] = ['foo']
df_out['B', 1] = ['bar']
sio_out = StringIO()
df_out.to_csv(sio_out)
sio_in = StringIO(sio_out.getvalue())
df_in = pd.read_csv(sio_in)

assert df_in.columns.tolist() == df_out.columns.tolist()

```

Cheers,
Alexis.
