to_datetime broken for Tz-aware datetimes and NaTs in same array
This works:

```
In [400]: 
pd.to_datetime([pd.Timestamp("2013-1-1", tz=pytz.timezone('US/Eastern'))])

Out[400]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-01-01 00:00:00-05:00]
Length: 1, Freq: None, Timezone: US/Eastern
```

but this (adding an NaT) does not. At the very least, the error message is misleading:

```
In [401]:
pd.to_datetime([pd.Timestamp("2013-1-1", tz=pytz.timezone('US/Eastern')), pd.NaT])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-401-3b5b8a67d2e1> in <module>()
----> 1 pd.to_datetime([pd.Timestamp("2013-1-1", tz=pytz.timezone('US/Eastern')), pd.NaT])

/usr/local/lib/python2.7/dist-packages/pandas-0.13.0rc1_78_g142ca62-py2.7-linux-x86_64.egg/pandas/tseries/tools.pyc in to_datetime(arg, errors, dayfirst, utc, box, format, coerce, unit)
    137         return Series(values, index=arg.index, name=arg.name)
    138     elif com.is_list_like(arg):
--> 139         return _convert_listlike(arg, box=box)
    140 
    141     return _convert_listlike(np.array([ arg ]), box=box)[0]

/usr/local/lib/python2.7/dist-packages/pandas-0.13.0rc1_78_g142ca62-py2.7-linux-x86_64.egg/pandas/tseries/tools.pyc in _convert_listlike(arg, box)
    127                 return DatetimeIndex._simple_new(values, None, tz=tz)
    128             except (ValueError, TypeError):
--> 129                 raise e
    130 
    131     if arg is None:

ValueError: Tz-aware datetime.datetime cannot be converted to datetime64 unless utc=True
```

I tracked down this issue to a bug in `datetime_to_datetime64`

When iterating over the elements in `datetime_to_datetime64` the check for nullness is `util._checknull(val)` which is `False` for `NaT`.

The correct null check is to use  `checknull` from  `lib`

PR on the way.
