ENH: Added stride/offset aliases in to_datetime
Added support for stride and offset aliases to the 'unit' keyword in
'to_datetime'.  Documented support for additional 'unit' intervals that
were available.

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