BUG/API: allow TimeGrouper with other columns in a groupby (GH3794)

closes #3794

```
In [3]: df
Out[3]: 
  Branch Buyer                Date  Quantity
0      A  Carl 2013-01-01 13:00:00         1
1      A  Mark 2013-01-01 13:05:00         3
2      A  Carl 2013-10-01 20:00:00         5
3      A  Carl 2013-10-02 10:00:00         1
4      A   Joe 2013-10-01 20:00:00         8
5      A   Joe 2013-10-02 10:00:00         1
6      A   Joe 2013-12-02 12:00:00         9
7      B  Carl 2013-12-02 14:00:00         3

[8 rows x 4 columns]

In [4]:    df.groupby([pd.Grouper(freq='1M',key='Date'),'Buyer']).sum()
Out[4]: 
                  Quantity
Date       Buyer          
2013-01-31 Carl          1
           Mark          3
2013-10-31 Carl          6
           Joe           9
2013-12-31 Carl          3
           Joe           9

[6 rows x 1 columns]

In [5]:    df = df.set_index('Date')

In [6]:    df['Date'] = df.index + pd.offsets.MonthEnd(2)

In [9]:    df.groupby([pd.Grouper(freq='6M',key='Date'),'Buyer']).sum()
Out[9]: 
                  Quantity
Date       Buyer          
2013-02-28 Carl          1
           Mark          3
2014-02-28 Carl          9
           Joe          18

[4 rows x 1 columns]

In [10]:    df.groupby([pd.Grouper(freq='6M',level='Date'),'Buyer']).sum()
Out[10]: 
                  Quantity
Date       Buyer          
2013-01-31 Carl          1
           Mark          3
2014-01-31 Carl          9
           Joe          18

[4 rows x 1 columns]

```