ENH: allow dict to name columns for multiple-aggregations in groupby
from SO:

http://stackoverflow.com/questions/22115671/apply-different-resampling-method-to-the-same-column-pandas

related #7700 (make sure to have name support)

Might be nice to support a dict for how on resample/groupby.agg so that the resulting
columns of an aggregation are named as the keys

`df.resample('A',how={ 'mymax' : 'max', 'mymean' : 'mean'})`

```
In [23]: df = DataFrame(np.random.randn(100,1),columns=['weight'],index=date_range('20000101',periods=100,freq='MS'))

In [24]: df.resample('A',how=['max','mean'])
Out[24]: 
              weight          
                 max      mean
2000-12-31  1.958570 -0.312230
2001-12-31  1.739518  0.035701
2002-12-31  2.503437  0.169365
2003-12-31  1.115315  0.149279
2004-12-31  2.190617 -0.087536
2005-12-31  1.286224  0.037669
2006-12-31  1.674017  0.147676
2007-12-31  2.107169 -0.064962
2008-12-31 -0.163863 -0.572363

[9 rows x 2 columns]
```
