Cleanup & De-duplication for custom offset classes
The diff looks a little bit mangled.  This PR does two things.

1) Centralize frequently-repeated calendar `__init__` in a new _CustomMixin

2) Merge a bunch of shared CustomBusinessMonthEnd and CustomBusinessMonthBegin logic into a parent class.

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