API: Groupby using a TimeGrouper missing methods
Groupbys from TGs seem to have some useful functionality suppressed/hidden (?)

```
import datetime as DT

df = pd.DataFrame({
'Branch' : 'A A A A A B'.split(),
'Buyer': 'Carl Mark Carl Joe Joe Carl'.split(),
'Quantity': [1,3,5,8,9,3],
'Date' : [
DT.datetime(2013,1,1,13,0),
DT.datetime(2013,1,1,13,5),
DT.datetime(2013,10,1,20,0),
DT.datetime(2013,10,2,10,0),
DT.datetime(2013,12,2,12,0),                                      
DT.datetime(2013,12,2,14,0),
]})

df = df.set_index('Date', drop=False)
g = df.groupby(pd.TimeGrouper('6M'))

In [104]: g.gr
g.group_keys  g.grouper     g.groups

In [104]: g.groups
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-104-b207e9b2ead3> in <module>()
----> 1 g.groups

/Users/234BroadWalk/pandas/pandas/core/groupby.pyc in __getattr__(self, attr)
    235
    236         raise AttributeError("'%s' object has no attribute '%s'" %
--> 237                              (type(self).__name__, attr))
    238
    239     def __getitem__(self, key):

AttributeError: 'DataFrameGroupBy' object has no attribute 'groups'
```

cc #3794
