DatetimeIndex should accept list of integers
brought up by #1263

```
In [5]: DatetimeIndex(ujson.decode(ujson.encode(rng)))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/wesm/code/pandas/<ipython-input-5-e33bac335db8> in <module>()
----> 1 DatetimeIndex(ujson.decode(ujson.encode(rng)))

/home/wesm/code/pandas/pandas/tseries/index.pyc in __new__(cls, data, freq, start, end, periods, copy, name, tz, verify_integrity, normalize, **kwds)
    227                 data = _str_to_dt_array(data, offset)
    228             else:
--> 229                 data = tools.to_datetime(data)
    230                 data = np.asarray(data, dtype='M8[ns]')
    231 

/home/wesm/code/pandas/pandas/tseries/tools.pyc in to_datetime(arg, errors, dayfirst)
     93         return lib.string_to_datetime(com._ensure_object(arg),
     94                                       raise_=errors == 'raise',
---> 95                                       dayfirst=dayfirst)
     96 
     97     try:

/home/wesm/code/pandas/pandas/_tseries.so in pandas._tseries.string_to_datetime (pandas/src/tseries.c:34314)()

TypeError: object of type 'long' has no len()
```
