apply over groups api for DataFrames
I am not sure what the right solution is, but 

```
dfext.head(10).groupby(['id','name']).apply(lambda g:  DataFrame({'i':[g.isCompatible.sum()],}).ix[0])
```

to get a 'select' over multiple variables seems wrong/fragile/complex.

I know we are busy here trying to reinvent sql badly, but maybe this should get sugar:

   dfext.groupby([vars]).select({k1:op1, k2:op2})   

where some magic can happen in there.  

Even simplifying down the _this is a one row data frame_ part would remove a lot of the snarl.

(Use case:  typically, I group, and I want multiple stats on multiple columns, or some stat that is a combination of multiple columns.... pandas.DataFrame is like `itertools.groupby` on steroids and with less hassle!)

It is possible of course that I might want Pivots or something.  If so, correct me.  
