astype attempts to convert datetime64[ms] as nanoseconds when missing value in data
When creating a DataFrame with a millisecond timestamp, created with `dtype='datetime64[ms]'`, this works as expected when there are no missing values in the data:

```
import pandas as pd
df = pd.DataFrame([1036713600000], dtype='float64')
print(df[0].astype('datetime64[ms]'))
```

Output:

```
0   2002-11-08
Name: 0, dtype: datetime64[ns]
```

Adding a missing value to the data causes the values to get parsed as nanoseconds rather than microseconds, which causes an exception:

```
df = pd.DataFrame([1036713600000, None], dtype='float64')
print(df[0].astype('datetime64[ms]'))
```

Output:

```
Traceback (most recent call last):                                          
  File "./f.py", line 6, in <module>                                        
    print(df[0].astype('datetime64[ms]'))                                   
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/generic.py", line 2411, in astype
    dtype=dtype, copy=copy, raise_on_error=raise_on_error, **kwargs)        
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 2504, in astype
    return self.apply('astype', dtype=dtype, **kwargs)                      
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 2459, in apply
    applied = getattr(b, f)(**kwargs)                                       
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 373, in astype
    values=values, **kwargs)                                                
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 407, in _astype
    fastpath=True, dtype=dtype, klass=klass)                                
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 2101, in make_block
    placement=placement)                                                    
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 1795, in __init__
    values = tslib.cast_to_nanoseconds(values)                              
  File "pandas/tslib.pyx", line 2622, in pandas.tslib.cast_to_nanoseconds (pandas/tslib.c:43295)
  File "pandas/tslib.pyx", line 1333, in pandas.tslib._check_dts_bounds (pandas/tslib.c:23332)
pandas.tslib.OutOfBoundsDatetime: Out of bounds nanosecond timestamp: 292278994-08-16 16:47:04
```

Expected output was something like:

```
0   2002-11-08
1   NaT
Name: 0, dtype: datetime64[ns]
```

This was using python 3.4.2, Pandas 0.16.2.
