Pandas doesn't accept dtype=np.datetime64
```
In [1]: from pandas import Series

In [3]: import numpy as np  

In [4]: Series([], dtype=np.datetime64)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-139e5646d234> in <module>()
----> 1 Series([], dtype=np.datetime64)

/users/is/whughes/pyenvs/1c4f50ee0e346415/lib/python2.7/site-packages/pandas-0.13.1-py2.7-linux-x86_64.egg/pandas/core/series.py in __init__(self, data, index, dtype, name, copy, fastpath)
    218             else:
    219                 data = _sanitize_array(data, index, dtype, copy,
--> 220                                        raise_cast_failure=True)
    221 
    222                 data = SingleBlockManager(data, index, fastpath=True)

/users/is/whughes/pyenvs/1c4f50ee0e346415/lib/python2.7/site-packages/pandas-0.13.1-py2.7-linux-x86_64.egg/pandas/core/series.py in _sanitize_array(data, index, dtype, copy, raise_cast_failure)
   2564 
   2565     else:
-> 2566         subarr = _try_cast(data, False)
   2567 
   2568     # scalar like

/users/is/whughes/pyenvs/1c4f50ee0e346415/lib/python2.7/site-packages/pandas-0.13.1-py2.7-linux-x86_64.egg/pandas/core/series.py in _try_cast(arr, take_fast_path)
   2509 
   2510         try:
-> 2511             arr = _possibly_cast_to_datetime(arr, dtype)
   2512             subarr = pa.array(arr, dtype=dtype, copy=copy)
   2513         except (ValueError, TypeError):

/users/is/whughes/pyenvs/1c4f50ee0e346415/lib/python2.7/site-packages/pandas-0.13.1-py2.7-linux-x86_64.egg/pandas/core/common.py in _possibly_cast_to_datetime(value, dtype, coerce)
   1659                 else:
   1660                     raise TypeError(
-> 1661                         "cannot convert datetimelike to dtype [%s]" % dtype)
   1662             elif is_timedelta64 and dtype != _TD_DTYPE:
   1663                 if dtype.name == 'timedelta64[ns]':

TypeError: cannot convert datetimelike to dtype [datetime64]
```

The following workaround works:

```
Series([], dtype=datetime)
```

but I would expect passing in the numpy type to work too.
