Add Series._repr_html_

<img width="454" alt="Screen Shot 2019-07-04 at 1 45 05 PM" src="https://user-images.githubusercontent.com/306380/60667707-13579000-9e62-11e9-88e6-ea1153057152.png">


- [x] closes #8829
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
