Series do not display HTML repr
Not sure if it's always been this way or if refactoring broke something.
In any case it looks strange when in ipnb:
![series repr](https://f.cloud.github.com/assets/1820866/1587251/87b19ace-5235-11e3-81d9-eabf7e342d53.png)

related to #5550, making reprs consistent.
