ohlc not available for groupby/etc
Was this available at some stage (it's looking in `self._cython_functions` for it rather than just saying no dice with a plain ol' AttributeError):

```
In [1]: rng = pd.date_range('2012', periods=3)

In [2]: df = pd.DataFrame([[1, 2], [1, 3], [5, 6]], rng, columns=['A', 'B'])

In [3]: g = df.groupby('A')

In [4]: g.foo()
AttributeError: 'DataFrameGroupBy' object has no attribute 'foo'

In [5]: g.ohlc()
KeyError: 'ohlc'

In [6]: g.ohlc(rule='D')
TypeError: ohlc() got an unexpected keyword argument 'rule'
```

Can do atm using `g.resample('D', how='ohlc')`.
