astype() for timedelta64 is broken in 0.12
The following code (or with any other frequency instead of ms):

```
t1 = pd.Series(pd.date_range('1/1/2011', periods=10, freq='H'))
t2 = pd.Series(pd.date_range('1/1/2011 01:02:03', periods=10, freq='H'))
x = t2-t1
x.astype('m8[ms]')
```

works well in pandas-0.11.0 (numpy-1.7.1), however raises an error in pandas-0.12.0:

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/vladimir/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/series.py", line 934, in astype
    values = com._astype_nansafe(self.values, dtype)
  File "/Users/vladimir/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/common.py", line 1653, in _astype_nansafe
    raise TypeError("cannot astype a timedelta from [%s] to [%s]" % (arr.dtype,dtype))
TypeError: cannot astype a timedelta from [timedelta64[ns]] to [timedelta64[ms]]
```

At the same time astype for a single element (which is numpy.timedelta64)

```
x[1].astype('m8[ms]')
```

obviously works well.
