ENH: Added 'sum' to the set of statistics returned by df.field.describe()
I've added ``sum`` to the set of statistics returned by the ``describe`` method.

I believe it should be added because in my experience, the sum is just as commonly needed as the other values already returned by ``describe``. I have often found myself writing an additional line of code after ``describe`` to calculate the sum of the same series. Adding sum to the mix would remove that requirement and make life easier for myself and others.

I believe I am not alone. For instance, ``sum`` is already included in the descriptive statistics methods of other programs, like Microsoft Excel.

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