ENH: `axis=None` option for statistical methods: `.min`, `.max`, `.median`, and so on
I am a long time user of pandas, thank you for all your hard work. 

#### Is your feature request related to a problem?
It would be useful to be able to do `df.min(axis=None)` rather than `df.values.min(axis=None)` to get the mimum over the whole DataFrame. The same goes for `max` and so on. I expected this to work since this is how `any` and `all` work.
#### Describe the solution you'd like
I'd like the default argument for `axis` to be `0` and then change the behaviour of `None` to do both axes, like numpy does. This would be then be consistent with `.all` and `.any`. 

Maybe changing behaviour of `axis=None` would be an issue, if so, maybe this would require the less desirable `axis="both"`.

#### API breaking implications
This would make the API more consitent, but would also be a breaking change. 

#### Describe alternatives you've considered
Writing `df.values.min(None)`.