ENH: resample to conjoin ohlc with other fields
Since ohlc produces a multi-index and other aggregations an index,
should promote the index to multi-index and conjoin

http://stackoverflow.com/questions/21140630/resampling-trade-data-into-ohlcv-with-pandas

e.g.
`df.resample('30s',how={'price' : 'ohlc', 'volume' : 'sum'})`
