ENH: Implement IntegerArray.sum
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I think this is mostly interesting in that it allows normalize=True for value_counts on an IntegerArray backed Series, which currently doesn't work:

```python
[ins] In [1]: s = pd.Series([1, 2, 3], dtype="Int64")

[ins] In [2]: s.value_counts(normalize=True)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-2bf1a78353e5> in <module>
----> 1 s.value_counts(normalize=True)

~/pandas/pandas/core/base.py in value_counts(self, normalize, sort, ascending, bins, dropna)
   1252             normalize=normalize,
   1253             bins=bins,
-> 1254             dropna=dropna,
   1255         )
   1256         return result

~/pandas/pandas/core/algorithms.py in value_counts(values, sort, ascending, normalize, bins, dropna)
    725
    726     if normalize:
--> 727         result = result / float(counts.sum())
    728
    729     return result

AttributeError: 'IntegerArray' object has no attribute 'sum'
```