DataFrame constructor raises exception with masked recarray
The `DataFrame` constructor supports numpy's ndarray of primitive type, recarray (or structured array) or a masked array of simple type. It breaks when given a masked recarray:

``` python
>>> a = np.ma.mrecords.fromrecords([(1,2)], names=['a', 'b'], mask=False)
>>> pandas.DataFrame(a)
...
TypeError: cannot perform reduce with flexible type
```

Tested at latest public release and HEAD.
