CLN, DOC: Remove unused parameters
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] remove unused parameters `out` and `dtype` from `cumulated` functions (`cummin`, `cumsum`, `cumprod`, `cummax`)

This also affects the documentation. Check for example: http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.cumsum.html, where the two parameters are mentioned, but not explained.
