BUG: various bug fixes for DataFrame/Series construction related to:
        0 and 1 len ndarrays
        datetimes that are single objects
        mixed datetimes and objects (GH #2751)
        astype now converts correctly with a datetime64 type to object, NaT are converted to np.nan
        _get_numeric_data with empty mixed-type returning empty, but index was missing
DOC: release notes updated, added missing_data section to docs