API: add top-level melt function as method
 - [x] (progress on?) #12640
   - [x] pd.melt (DataFrame) (xref #14876)

 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

This is continued from [PR#15513](https://github.com/pandas-dev/pandas/pull/15513).