BUG: TimeGrouper outputs different result by column order
closes #6764

`TimeGrouper` may output incorrect results depending on the target column order. The problem seems to be caused by 2 parts.
- `TimeGrouper._get_time_bins` and related methods expects sorted values input.
- `BinGrouper.get_iterator` expects sorted data input. 

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

# correct
>>> df.groupby([pd.Grouper(freq='1M',key='Date'),'Buyer']).sum()
                  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]

>>> df_sorted = df.sort('Quantity')          # change "Date" column unsorted
# incorrect
>>> df_sorted.groupby([pd.Grouper(freq='1M',key='Date'),'Buyer']).sum()
                  Quantity
Date       Buyer          
2013-01-31 Carl          1
2013-10-31 Carl          1
           Joe           1
           Mark          3
2013-12-31 Carl          8
           Joe          17

[6 rows x 1 columns]

>>> df_sorted.groupby([pd.Grouper(freq='1M',key='Date', sort=True),'Buyer']).sum()
# same incorrect result
```

```
# correct
>>> df.groupby([pd.Grouper(freq='6M',key='Date'),'Buyer']).sum()
                  Quantity
Date       Buyer          
2013-01-31 Carl          1
           Mark          3
2014-01-31 Carl          9
           Joe          18

[4 rows x 1 columns]

# incorrect
>>> df_sorted.groupby([pd.Grouper(freq='6M',key='Date'),'Buyer']).sum()
                  Quantity
Date       Buyer          
2013-01-31 Carl          1
2014-01-31 Carl          9
           Joe          18
           Mark          3

[4 rows x 1 columns]
```

this close #6764 ?
 It seems do. I've commented on #6764, and will add a test for this.
 just run a quick perf check - anything shows up post and investigate

almost certainly fine - good to check anyhow
 Can u add in a release note

I think their was a releated issue u can simply add this onto (for pd.Grouper) (or after)
 Thanks, I've added release note and test for #6764.

My initial implementation had unnecessary sort and it makes time series performance worse. Now it is fixed and following is an updates vbench result. No test looks consntantly get worse.

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
....
indexing_dataframe_boolean_rows_object       |   1.0853 |   0.9326 |   1.1637 |
frame_ctor_dtindex_YearEnd(2)                |   0.9511 |   0.8060 |   1.1799 |
write_store_table_mixed                      |  76.4666 |  64.6210 |   1.1833 |
indexing_dataframe_boolean                   |  24.6797 |  20.7040 |   1.1920 |
eval_frame_and_all_threads                   |  41.8170 |  30.7407 |   1.3603 |
-------------------------------------------------------------------------------
```
