POC for New GroupBy Dispatching Module
This is nowhere near completion but looking for feedback on the direction.

@jreback 

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

if you can move underneath pandas/core/groupby then can have a look Closing for now - going to take an additional pass at a later date