data frame and custom series class
Pandas data frame does not respect custom series class of a column.

For example

```
>>> import pandas
>>> class XSeries(pandas.Series): pass
>>> df = pandas.DataFrame({'a': XSeries([1, 2, 3]), 'b': pandas.Series([3, 4, 5])})
>>> type(df.a)
pandas.core.series.Series
```

I would expect type(df.a) to be XSeries in the example above.
