Write fast OHLC aggregator

[wesm]

To work with GroupBy

[wesm]

speed of current OHLC function can be improved:

def translate_grouping(how):
    if set(how) == set('ohlc'):
        return {'open'  : lambda arr: arr[0],
                'low'   : lambda arr: arr.min(),
                'high'  : lambda arr: arr.max(),
                'close' : lambda arr: arr[-1]}

    if how in 'last':
        def picker(arr):
            return arr[-1] if arr is not None and len(arr) else np.nan
        return picker

    raise ValueError("Unrecognized method: %s" % how)