ENH: add quantiles kw to Series.describe to create dynamic quantiles
This of course can be done directly, but adding the keyword `quantiles` that accepts a number or list (to create multiple fields), seems like a nice idea

and maybe deprecate `percentile_width` / replace with `quantiles=[50]`

like this:
http://stackoverflow.com/questions/17578115/pass-percentiles-to-pandas-agg-function
