API: add top-level functions as method
 - [ ] closes #12640
   - [x] pd.crosstab (DataFrame)
   - [x] pd.melt (DataFrame) (xref #14876)
   - [x] pd.pivot_table (might need a new name as would be confusing with DataFrame.pivot)
   - [x] pd.get_dummies (both)
   - [x] pd.cut/qcut (only on Series)

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