Incorrect handling of datetime64 values in structured arrays
see:

```
arr = np.array([ (datetime.datetime(2012, 9, 9, 0, 0), datetime.datetime(2012, 9, 8, 15, 10))],
dtype=[('Date', '<M8[us]'), ('Forecasting', '<M8[us]')])
DataFrame(arr)
```

cf http://stackoverflow.com/questions/12369546/pandas-parsing-datetime-column-from-sqlite-database
