move pivot_table doc-string to DataFrame
instead of pinning it on over pin `core.reshape.pivot`

At the moment `core.reshape.pivot.pivot_table` is pinned to `DataFrame` by writing `DataFrame.pivot_table = pivot_table`.  This PR defines `pivot_table` directly in `DataFrame` and then calls to the function.  It's the same pattern already used by `DataFrame.pivot`.

I think this is the last `DataFrame` method pinned on to `DataFrame` this way.  Two more left in `Series`

 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [ ] whatsnew entry
