API cleanup: Remove global duplicates of DataFrame methods
In the currently online pandas 0.19.2 documentation, I see the following inconsistency:

* There is [pandas.DataFrame.pivot_table](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.pivot_table.html), but also [pandas.pivot_table](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.pivot_table.html).
* In the same way, there is [pandas.DataFrame.pivot](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.pivot.html) and also [pandas.pivot](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.pivot.html). (BTW., the latter has the dataframe argument missing.)
* There is [pandas.melt](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.melt.html), but no pandas.DataFrame.melt.

The global versions of these methods (and probably others) are identical to the local ones except for an additional DataFrame argument, I think that all these should only be DataFrame methods. This would e.g. consistently enable chaining.

So let me suggest that the global versions should be removed, of course after a deprecation period.
