ENH: add to cython groupby functions
- [x] shift [here](http://stackoverflow.com/questions/17401197/how-to-speed-up-pandas-multilevel-dataframe-shift-by-group) and [here](http://stackoverflow.com/questions/25298659/faster-alternative-to-grouby-shift)
- [x] [cumsum](http://stackoverflow.com/questions/28453238/cumsum-in-pandas-groupby-is-slow)
- [x] [cumprod](http://stackoverflow.com/questions/32138067/quick-pandas-groupby-calculations)
