timeseries related .groups bug
related to #1423

```
from pandas import *                                                                                  
import numpy as np                                                                                    
periods = 1000                                                                                        
ind = DatetimeIndex(start='2012/1/1', freq='5min', periods=periods)                                   
df = DataFrame({'high': np.arange(periods), 'low': np.arange(periods)}, index=ind)                    

grouped = df.groupby(lambda x: datetime(x.year, x.month, x.day))      



In [9]: grouped.groups

ValueError                                Traceback (most recent call last)
<ipython-input-9-aec229cf123a> in <module>()
----> 1 grouped.groups

~/pandas/pandas/core/groupby.pyc in groups(self)
    150     @property
    151     def groups(self):
--> 152         return self.grouper.groups
    153 
    154     @property

~/pandas/pandas/core/index.pyc in groupby(self, to_groupby)
    740 
    741     def groupby(self, to_groupby):
--> 742         return self._groupby(self.values, to_groupby)
    743 
    744     def map(self, mapper):

...
...
...

~/pandas/pandas/lib.so in pandas.lib.groupby_arrays (pandas/src/tseries.c:67246)()

ValueError: Buffer dtype mismatch, expected 'int64_t' but got Python object
```
