DataFrame.from_records doesn't handle missing dates (None)
When you construct a DataFrame from a numpy recarray with datetime data and None for missing dates you get an error.

``` python
arrdata = [np.array([datetime.datetime(2005, 3, 1, 0, 0), None])]
dtypes = [('EXPIRY', '<M8[m]')]
recarray = np.core.records.fromarrays(arrdata, dtype=dtypes)

df = pd.DataFrame.from_records(recarray)
Traceback (most recent call last):
  File "/users/is/dbew/pyenvs/timeseries/lib/python2.7/site-packages/ipython-1.1.0_1_ahl1-py2.7.egg/IPython/core/interactiveshell.py", line 2830, in run_code
    exec code_obj in self.user_global_ns, self.user_ns
  File "<ipython-input-33-ae01f48c3b82>", line 1, in <module>
    df = pd.DataFrame.from_records(recarray)
  File "/users/is/dbew/pyenvs/timeseries/lib/python2.7/site-packages/pandas-0.13.0_408_g464c1f9-py2.7-linux-x86_64.egg/pandas/core/frame.py", line 841, in from_records
    columns)
  File "/users/is/dbew/pyenvs/timeseries/lib/python2.7/site-packages/pandas-0.13.0_408_g464c1f9-py2.7-linux-x86_64.egg/pandas/core/frame.py", line 4473, in _arrays_to_mgr
    return create_block_manager_from_arrays(arrays, arr_names, axes)
  File "/users/is/dbew/pyenvs/timeseries/lib/python2.7/site-packages/pandas-0.13.0_408_g464c1f9-py2.7-linux-x86_64.egg/pandas/core/internals.py", line 3748, in create_block_manager_from_arrays
    construction_error(len(arrays), arrays[0].shape[1:], axes, e)
  File "/users/is/dbew/pyenvs/timeseries/lib/python2.7/site-packages/pandas-0.13.0_408_g464c1f9-py2.7-linux-x86_64.egg/pandas/core/internals.py", line 3720, in construction_error
    passed,implied))
ValueError: Shape of passed values is (1,), indices imply (1, 2)
```

This is a regression from 0.11.0. Stepping through the code it looks the initial error is raised in tslib.cast_to_nanoseconds and then caught and re-raised in create_block_manager_from_arrays

Incidentally, construction does work from a simple array instead of a recarray:

``` python
pd.DataFrame(np.array([datetime.datetime(2005, 3, 1, 0, 0), None]))
Out[36]: 
           0
0 2005-03-01
1        NaT

[2 rows x 1 columns]
```
