ENH: Tweaks to axis handling (esp for flex methods)
- [x] Add 'rows' as synonym for 'index' for DataFrame and Series - #5309
- [x] Allow Series to accept axis kwarg for flex ops (and only allow axis=0) - #5352
- [x] Fix Panel functions that aren't getting axis correctly - #5354

I think (2) may already be there, but I'm not sure and I want to check that.

This should be really trivial to do but it should make interactive playing around with frame and series a bit more intuitive. Don't think it makes sense for panel.
