Categorical: let unique only return used categories
Discussion see here: https://github.com/pydata/pandas/issues/8559#issuecomment-64189570

put a release note referincing that issue. I think this is already in the API.rst ? (maybe just check doc-string too).
 `Categorical.unique` is not in api.rst, only methods under `Series.cat` are.

re release note: under API change or bug fix?
 ahh, so maybe add to `Series.cat` methods? I forgot how we are calling this

`s.unique()` of course does it, but I suppose `s.cat.unique()` should be allowed as well?

This 'changes' the API slightly so API change it is. (maybe put a short explanation), example if you feel it is waranted.
 ok, added it under api change (without an example)

I don't think any more methods should be added to `Series.cat`: simple calling `s.unique()` (or `s.min()` or `s.describe()`) is enough. After the last discussion about what is API and what is not, I started to think of all methods in `Categorical` as "for compatibility with numpy" and I think that's the right way to think about them. If a users knows better they can look at the docstrings, but I don't think pandas should do any API guarantee whether these methods stay or change or be removed. 
 thanks!
