GroupBy using TimeGrouper does not work
BUG: TimeGrouper not too friendly with other groups, e.g.

`df.set_index('Date').groupby([pd.TimeGrouper('6M'),'Branch']).sum()`
should work

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Hi everybody,

I found two issues with TimeGrouper:

1) TimeGrouper does not work at all:

Let's take the following example:

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(2013,10,3,10,0),
        DT.datetime(2013,12,2,12,0),  
        DT.datetime(2013,12,2,14,0),
        ]})

gr = df.groupby(pd.TimeGrouper(freq='6M'))

def testgr(df):
    print df

gr.apply(testgr)

This will raise the Exception: "Exception: All objects passed were None"

2) With previous Panda's version it was not possible to combine TimeGrouper with another criteria such as "Branch" in my case. 

Thank you very much

Andy
