Adding Series._repr_html_
This is intended to close #5563 by adding `Series._repr_html_`
As suggested by previous discussion, the HTML representation of a Series looks different from the representation of a DataFrame, specifically by omitting the `<thead>` row, which (for a DataFrame) contains the column names.
I've tested it in a Jupyter notebook; I can provide a screenshot if someone suggests the best way to do that.


- [ ] closes #5563
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
 - [ ] whatsnew entry
