PERF: speed up multi-key groupby
Improves multi-key `groupby` speed; On master:

```
In [3]: pd.__version__
Out[3]: '0.14.1-276-g995f91c'

In [4]: np.random.seed(2718281)

In [5]: n = 20000

In [6]: df = pd.DataFrame(np.random.randint(1, n, (n, 3)),
   ...:         columns=['jim', 'joe', 'jolie'])

In [7]: %timeit df.groupby(['jim', 'joe'])['jolie'].transform('max')
1 loops, best of 3: 1.09 s per loop

In [8]: df['joe'] = df['jim']

In [9]: %timeit df.groupby(['jim', 'joe'])['jolie'].transform('max')
1 loops, best of 3: 1.02 s per loop
```

note that it is not responsive to the reduction in number of groups. With this patch:

```
In [9]: %timeit df.groupby(['jim', 'joe'])['jolie'].transform('max')
10 loops, best of 3: 122 ms per loop

In [10]: df['joe'] = df['jim']

In [11]: %timeit df.groupby(['jim', 'joe'])['jolie'].transform('max')
10 loops, best of 3: 82.3 ms per loop
```

benching:

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_transform_multi_key2                 |  48.2820 | 810.2150 |   0.0596 |
groupby_transform_multi_key4                 | 137.5050 | 1986.8030 |   0.0692 |
groupby_transform_multi_key1                 |  70.1749 | 841.8140 |   0.0834 |
groupby_transform_multi_key3                 | 713.8393 | 2613.6247 |   0.2731 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [11cc057] : Merge branch 'groupby-speed-up' of https://github.com/behzadnouri/pandas into behzadnouri-groupby-speed-up
Base   [3bb0803] : Merge pull request #8103 from sinhrks/pivot_dt

BUG: pivot_table raises KeyError with nameless index and columns
```

- need a vbench (or 2) (and then show the results)
- release note
 I had some dependency issue, so ran the benchmarks manually;
On master:

```
>>> groupby_transform_multi_key1.run()
{'loops': 1, 'timing': 1880.3930282592773, 'repeat': 3, 'succeeded': True, 'units': 'ms'}
>>> groupby_transform_multi_key2.run()
{'loops': 1, 'timing': 1886.4881992340088, 'repeat': 3, 'succeeded': True, 'units': 'ms'}
>>> groupby_transform_multi_key3.run()
{'loops': 1, 'timing': 5588.175058364868, 'repeat': 3, 'succeeded': True, 'units': 'ms'}
>>> groupby_transform_multi_key4.run()
{'loops': 1, 'timing': 4688.298940658569, 'repeat': 3, 'succeeded': True, 'units': 'ms'}
```

on branch:

```
>>> groupby_transform_multi_key1.run()
{'loops': 1, 'timing': 106.5061092376709, 'repeat': 3, 'succeeded': True, 'units': 'ms'}
>>> groupby_transform_multi_key2.run()
{'loops': 10, 'timing': 73.46320152282715, 'repeat': 3, 'succeeded': True, 'units': 'ms'}
>>> groupby_transform_multi_key3.run()
{'loops': 1, 'timing': 1123.687982559204, 'repeat': 3, 'succeeded': True, 'units': 'ms'}
>>> groupby_transform_multi_key4.run()
{'loops': 1, 'timing': 200.47903060913086, 'repeat': 3, 'succeeded': True, 'units': 'ms'}
```

`groupby_transform_multi_key[3|4]` needs 6.5G memory to run on master.
 why is the memory usage so high? the Cartesian product of the groups is not represented here (it's only the compressed space)
 The master branch calls into [groupsort_indexer](https://github.com/pydata/pandas/blob/master/pandas/algos.pyx#L1980) with [`ngroups = np.prod(shape)`](https://github.com/pydata/pandas/blob/master/pandas/core/groupby.py#L3550). `np.prod(shape)` is the size of Cartesian product space of unique values across each key and `groupsort_indexer` allocates an array `counts` with this size.

[Elsewhere](https://github.com/pydata/pandas/blob/master/pandas/core/groupby.py#L3434) also, the code falls back on argsort to avoid memory error.
 `argsort` says is O(n**2) (as the default is quicksort)....but only using it on the smaller ones anyhow.
thanks..this is great! (and the mem issue is avoided!)
 thanks! this was great!
 @jreback On further tests, it seems to me that we need a _stable_ sorter for `group_index`. see for example
- [tests/test_groupby.py:test_lexsort_indexer](https://github.com/pydata/pandas/blob/master/pandas/tests/test_groupby.py#L4488)
- [tests/test_frame.py:test_sort_index_multicolumn](https://github.com/pydata/pandas/blob/master/pandas/tests/test_frame.py#L10322)
  It compares versus `np.lexsort` which is a stable sort.
- [tests/test_groupby:test__cython_agg_general](https://github.com/pydata/pandas/blob/master/pandas/tests/test_groupby.py#L4488):
  `first` fails without stable sort.

I need to change the code to `.argsort(kind='mergesort')`.  Should I make a new pull-request or it can work within this pull-request? 

I did some tests with merge-sort and benchmarks still look good. It is also inline with the fact that Wes uses merge sort in [here](https://github.com/pydata/pandas/blame/master/pandas/core/groupby.py#L3436).
 ok make a new pr

hmm no tests break
can u show a test case?
