ENH: Harmonize drop and rename API
`rename` accepts a `columns` argument or an `index` argument, while `drop` looks for a `labels` and `axis` pair. I don't know about anyone else, but I have to check the help file every time I come back to pandas to remember which takes which. 

How would people feel about adding `columns` and `index` arguments to `drop`? They could just be added in addition to `labels`/`axis` if we want to provide backwards compatibility and just raise an exception if the user tries to mix them. 
