ENH: groupby().apply(f) accepts raw arg, to return results unmolested #3241

#3241
Right now:
```python

In [16]: df=mkdf(10,2,data_gen_f=lambda x,y: randint(1,10))
    ...: df
    ...: 
    ...: 
    ...: 
Out[16]: 
C0       C_l0_g0  C_l0_g1
R0                       
R_l0_g0        9        1
R_l0_g1        3        7
R_l0_g2        8        1
R_l0_g3        4        3
R_l0_g4        5        3
R_l0_g5        7        2
R_l0_g6        4        1
R_l0_g7        5        4
R_l0_g8        9        7
R_l0_g9        4        8

In [17]: def f1(g):
    ...:     return g.sort('C_l0_g0')
    ...: # group on the suffix of the running index 
    ...: g=df.groupby(lambda key: int(key.split("g")[-1]) >= 5)
    ...: r=g.apply(f1)
    ...: 

# we want to return each group dataframe sorted, but we get concatted against our will
In [18]: r
Out[18]: 
C0             C_l0_g0  C_l0_g1
      R0                       
False R_l0_g1        3        7
      R_l0_g3        4        3
      R_l0_g4        5        3
      R_l0_g2        8        1
      R_l0_g0        9        1
True  R_l0_g6        4        1
      R_l0_g9        4        8
      R_l0_g7        5        4
      R_l0_g5        7        2
      R_l0_g8        9        7

# what we want really, is a couple of sorted dataframes:

In [20]: map(lambda r: r[1].sort('C_l0_g0'),g)
Out[20]: 
[C0       C_l0_g0  C_l0_g1
R0                       
R_l0_g1        3        7
R_l0_g3        4        3
R_l0_g4        5        3
R_l0_g2        8        1
R_l0_g0        9        1,
 C0       C_l0_g0  C_l0_g1
R0                       
R_l0_g6        4        1
R_l0_g9        4        8
R_l0_g7        5        4
R_l0_g5        7        2
R_l0_g8        9        7]
```

#### With this PR:

```python
In [21]: def f1(g): # same f1 as above
    ...:     return g.sort('C_l0_g0')
    ...: def f2(g,raw=None):
    ...:     return g.sort('C_l0_g0')
    ...: def f3(g,**kwds):
    ...:     return g.sort('C_l0_g0')
    ...: # the  `raw` keyword is the new bit
    ...: r1=g.apply(f1,raw=True)
    ...: r2=g.apply(f2,raw=True)
    ...: r3=g.apply(f2,raw=True)
    ...: 
# a bunch of sorted frames
In [22]: print r1
[(False, C0       C_l0_g0  C_l0_g1
R0                       
R_l0_g1        3        7
R_l0_g3        4        3
R_l0_g4        5        3
R_l0_g2        8        1
R_l0_g0        9        1), (True, C0       C_l0_g0  C_l0_g1
R0                       
R_l0_g6        4        1
R_l0_g9        4        8
R_l0_g7        5        4
R_l0_g5        7        2
R_l0_g8        9        7)]

# but not if the transformer function signature uses **kwds, or 'raw' already
In [23]: print r2
C0             C_l0_g0  C_l0_g1
      R0                       
False R_l0_g1        3        7
      R_l0_g3        4        3
      R_l0_g4        5        3
      R_l0_g2        8        1
      R_l0_g0        9        1
True  R_l0_g6        4        1
      R_l0_g9        4        8
      R_l0_g7        5        4
      R_l0_g5        7        2
      R_l0_g8        9        7

In [24]: print r3
C0             C_l0_g0  C_l0_g1
      R0                       
False R_l0_g1        3        7
      R_l0_g3        4        3
      R_l0_g4        5        3
      R_l0_g2        8        1
      R_l0_g0        9        1
True  R_l0_g6        4        1
      R_l0_g9        4        8
      R_l0_g7        5        4
      R_l0_g5        7        2
      R_l0_g8        9        7
```
```