ENH: Allow quantile to accept an int
This change is to allow `quantile` to accept an `int` argument to make it easier to compute a set of evenly-spaced quantiles.  For example, instead of typing `s.quantile([0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9])` or using `np.linspace` you can type `s.quantile(10)`.

- [ - ] closes #xxxx (no issue)
- [ x ] tests added / passed
- [ x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ x ] whatsnew entry