ENH ohlc resample for DataFrame
closes #2320

Can use ohlc from DataFrame.

cc @jreback

Example:

```
In [24]: df = pd.DataFrame({'PRICE': {Timestamp('2011-01-06 10:59:05', tz=None): 24990,
  Timestamp('2011-01-06 12:43:33', tz=None): 25499,
  Timestamp('2011-01-06 12:54:09', tz=None): 25499},
 'VOLUME': {Timestamp('2011-01-06 10:59:05', tz=None): 1500000000,
  Timestamp('2011-01-06 12:43:33', tz=None): 5000000000,
  Timestamp('2011-01-06 12:54:09', tz=None): 100000000}})

In [25]: df.resample('H', how='ohlc')
Out[25]: 
                     PRICE                           VOLUME              \
                      open   high    low  close        open        high   
2011-01-06 10:00:00  24990  24990  24990  24990  1500000000  1500000000   
2011-01-06 11:00:00    NaN    NaN    NaN    NaN         NaN         NaN   
2011-01-06 12:00:00  25499  25499  25499  25499  5000000000  5000000000   


                            low       close  
2011-01-06 10:00:00  1500000000  1500000000  
2011-01-06 11:00:00         NaN         NaN  
2011-01-06 12:00:00   100000000   100000000 
```

no what puzzles me is why ohlc fails and describe almost works
(well ohlc is a cython function and describe is not) so there is a disconnect that allows one path to work (almost) and the other to fail
 @jreback  What did you think about this one? Not sure what we were looking into re describe (is that a separate issue*?)

\* _describe should have MultiIndex column, rather than index._
 can you put a test in for doing the same with describe and see what happens?
 When I did this last time and also in master:

```
In [29]: df.groupby('PRICE').describe()  # expected .unstack(1)
Out[29]: 
             PRICE        VOLUME
PRICE                           
24990 count      1  1.000000e+00
      mean   24990  1.500000e+09
      std      NaN           NaN
      min    24990  1.500000e+09
      25%    24990  1.500000e+09
      50%    24990  1.500000e+09
      75%    24990  1.500000e+09
      max    24990  1.500000e+09
25499 count      2  2.000000e+00
      mean   25499  2.550000e+09
      std        0  3.464823e+09
      min    25499  1.000000e+08
      25%    25499  1.325000e+09
      50%    25499  2.550000e+09
      75%    25499  3.775000e+09
      max    25499  5.000000e+09
```

so, it appends it to index, rather than as a MultiIndex column,...
 hmm...must be because the `ohlc` is a cythonized and the `describe` is not (so it a general groupby). I think what you show as the ohlc is correct, so then I guess that this a a bug (but different)
 so merge? opened issue re describe
 ok 
